Yes, I've noticed that. I think it's kinda silly and I'll explain why.
From what I've seen of comp LD games since armlet bear became a thing, I've noticed a few things.
1. Both LD and his bear don't actually do anything unless something is rooted. Of course, they still destroy towers just as fast but in terms of teamfights and ganks and counter-ganks, they are absolutely useless unless their target is stunned somehow.
2. Armlet bears tend to get 6-slotted while LD himself runs around with just boots.
Regarding #1:
There's a big reason why melee heroes can't outcarry ranged heroes unless they have big AOE abilities such as cleave. It's because the effective amount of autoattack hits for a melee hero is significantly less than ranged. For example, for each hit Sven can pull off, Drow could easily have pulled off two or three. Furthermore, it's impossible to damage things while moving as a melee carry, something ranged heroes can do as well.
Why is this an issue? I say this because this means the stats and bonuses earned from armlet (and basically every dps item) is deceptive on a melee hero. What you see is potential damage. It doesn't account for the hero getting kited or the amount of time spent doing 0 dps because of movement. To further illustrate my point, observe Ursa. He also can't do anything unless he's right next to his target and they can't run away. So even if an Ursa has three hyperstones, he will still do effectively 0 dps if he cannot hit his target. To a less extreme example, even if he has three hyperstones, he will only have approximately 1 attack per second if his target is the same speed as him. Remember, him having to catch up to his enemy means less dps total.
In this case, both LD and his bear have the exact same issue as Ursa does, except the bear has a root. However, unlike Ursa, these guys don't have the ability to make an enemy explode in 1 second so they need their target to be in range for much... much longer. Which is what I meant by they can't do anything unless the target is stunned. So even if Armlet gave 100% attack speed, it will still be limited by how many attacks the bear can get before the target moves out of melee range and the bear has to reposition.
Radiance bear doesn't have this issue. Radiance dps is constant and guaranteed unless the enemy has magic immunity. So all the bear has to do is be somewhat near the enemy and it will proc the full amount of damage regardless. In this case the bear can do the "run next to enemy. Stop to attack. Run next to enemy. Stop to attack" thing while still doing full damage. I personally think this is a massive advantage and what makes or breaks LD. This sentiment will be echoed in the next point.
Regarding #2:
In economics there is a theory called opportunity cost. What it basically means is that there is a hidden cost for all our choices. For example, suppose at this moment I have a choice of whether to write this article or not. If I write it, the opportunity cost of that action is I just spent 1/2 hour not cramming for my life insurance and investments certification exam. Similarly, if I don't write it, the opportunity cost is you guys don't get the pleasure of reading a massive and really boring wall of text.
In terms of Lone Druid, what this means is simple. If you notice, LD games in the past where the bear gets a Radiance fast tend to only have 2 items on the bear, and the rest going on LD himself. This tends to be Radiance + AC / Mjolnir. After that LD starts getting Heart, Shivas, whatever. Recently with the armlet bear, what you see is LD stacking 5 mid-priced items + AC / Mjolnir on the bear instead. So you'll see a bear with something like Armlet, Vlads, Basher, AC etc. and LD just running around with boots. Essentially, the opportunity cost for not getting Radiance is that LD effectively spends more gold in the long run to make a bear just as effective as a Radiance build bear. Those small items all add up and it leaves LD himself relatively vulnerable since he has no items.
Why exactly does this happen? Why does Radiance bear only need two items? This is because as I've mentioned in the first point, Radiance bear can output amazing damage just by being there. It doesn't need a Basher to force enemies to sit still and get shredded. It doesn't need a Vlads to lifesteal and amplify damage while it DPS-es something down. Radiance bear has been known to not even need upgraded boots as the Radiance AOE is so huge. Comparatively, armlet LD has to get a bunch of items to supplement the bear just to reach the level Radiance bear gets by default. What is the benefit of Armlet bear? Simply that the incremental stages are all very appealing as compared to the gigantic steps of 3800g for the Sacred Relic (which on its own is kinda derpy).
Additionally, this also means that when the bear inevitably dies, armlet LD is significantly weaker than Radiance LD since he will have less items on himself.
Why is the armlet build so appealing right now?
As you mentioned. Armlet + Hyperstone is significantly cheaper than a Radiance. Right now a lot of pro teams like more cost effective items that work now as opposed to big items that work "possibly in the future". This means items like Drums, Mek, and Ring of Aquila are all loved more than saving up for big stuff like AC. Generally I agree with this sentiment, which is why I think Radiance on most heroes is kinda dumb.
Furthermore, the increased bonuses from the Armlet build help out in tower pushing quite a lot. After all, towers can't run away so the bear gets the full amount of dps. This is very compelling for push oriented lineups, which is generally why LD is picked.
Lastly, I think people are being seduced by the prospect of the bear not taking the Armlet active damage. Personally I think this is quite a silly sentiment, as I'm not particularly willing to pay 2600g for 40 damage and 25 attack speed, even if it didn't drain my hp constantly. I think if people valued that side of Armlet so much, you'd see Armlet on all the right click heroes in the game. Drow, Luna, and Gyro would all be getting it. Hell, they'd even get the gigantic HP boost and the ability to armlet toggle too, something that the bear doesn't get. But this isn't the case.
Essentially you have conversations like this:
A: "Holy crap! LD's bear doesn't take Armlet damage!"
B: "Wow, that means he gets all the upsides without any of the downsides!"
Except that isn't true. Since he doesn't get the massive strength boost. 25 strength is the same as Reaver or two Vitality Boosters. He also doesn't get the armlet toggle, which is a vital component of making good use of Armlet. Even being conservative, I think forgoing this part removes 50% of the item's effectiveness. That's the true price of not having the active drain.
In conclusion:
I think Armlet bear is probably better as a backup strategy, but Radiance bear just has far more advantages, even if it's harder to identify. Not to say Armlet bear is bad, because it definitely destroys towers faster. Rather it's just in a fad phase right now.
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Edit: By the way, if you're wondering how much damage exactly Radiance does, if the bear is alive in a fight for 10 seconds, he'd have done as much damage as Zeus's level 3 ultimate.