Whoah there partner, don't you go dissin' on Treant... Strength-based hero with the second highest strength gain in the game, and natural attack damage outmatched only by Tiny, with on-demand low-cooldown soft disables and a combat invis/movespeed boost? The guy can actually build to be a semicarry/more with the right items, and in contrast to other carries/semis he's brilliant early game. If he chooses to farm, his laning phase is a cinch due to his great health and attack damage for last-hitting. If things get hairy, head for the trees.
Yes, his abilities are most certainly the most use early on, when the regen/lifesteal values are high enough to mean something. But I think people may underestimate the power of Living Armor later on. Due to the damage block, at max level it's a free 560 effective combat health on a 15-second cooldown, plus a teensy bit of regen. It makes a difference.
Just like how you shouldn't underestimate Shadow Demon being able to shove the beefed-up enemy carry out of the fight for 2.5 seconds, after which two high-powered illusions (look at the percentages they get, compared to other stuff like Manta style or Siren's Mirror Image) pop up and start tearing apart their origin. Great fun on anyone who thought to dominate using a Diffusal blade.
And yeah, while Illuminate does do a lot of damage, and does it early on, it is by no means without its faults... The high mana cost means you basically need to hog a lot of your own Chakras when it's up, and the channeling time requires either some incredible planning/luck, or some very strong disables courtesy of somebody else. Sure, it will help to wear down the enemies and possibly let your carry get a gank or two, but it also pushes and can potentially steal the kill... Whereas Mana Leak simply drains and/or stuns enemies so they can't run away or effectively fight back while your carry mashes them to bits. And yeah, KotL and AM can make a very fun pair, at least as soon as AM gets his ultimate up and running.