As someone who just designed and built my first frame quarry from scratch, I have to tell you that you seem to be getting them wrong. First of all, they require a MASSIVE initial investment of time, effort and resources,
Eh, I have to disagree. The resource investment is higher than a BC quarry but they're not terribly difficult to make at all. I've made about a dozen of them now and I can probably knock a simple one together within the first 2 hours of gameplay now. If I find a bounty of diamonds right away I can just make a BC quarry to provide resources for the frame quarry. Either way, once the frame quarry is finished there is no user interaction required to gather more resources.
and the rewards they bring are proportional to that investment
The rewards are proportional to the time you leave minecraft running while you are sitting in a safe room or inside your frame contraption waiting for the 1000 cycle mining program to finish and loop.
When I built my quarry, it took pretty much all of my materials, plus a good deal of extra mining to build, and about a week of design and redesign, and it is certainly not capable of mining indefinitely yet.
I don't mean to be rude but making a frame quarry which can mine indefinitely is really quite easy. They don't require fuel, just sunlight or lava/water. Or even an enderchest based battery system.
Why did I go through all the trouble of building it if not for the resources it can mine? Because frame quarries (and framecraft in general) are cool and fun to make. When I play minecraft, I don't do it to mine all the diamonds in the world and laugh at how rich I am, I do it to make cool stuff. And a big reason I play multiplayer is to show off my cool stuff and marvel at other peoples cool stuff.
I agree completely, it is fun to show off the crazy stuff you build in multiplayer.
I also think that if you make framecraft that much harder to obtain you'll be missing a huge opportunity for interesting PVP involving them. I would love to see aerial frame battle-platforms bombing towns from above, and also the countermeasures that get put in place to stop them.
I had thought about that, and it would indeed be quite a lot of fun. That's one reason I won't be disabling frames and frame equipment. I do, however, think they should be special and rare, not knocked together on a whim and thrown into battle. It should hurt when your airship is destroyed, not feel like "Eh I'll just wait till my frame quarry digs out another 12 chunks and make two more airships whatever"
If "AFK-mining" is really such a huge problem, why not just make World Anchors uncraftable and only available through admin stores, instead of the frame materials? That should discourage anyone wanting to abuse quarries, while still allowing people to build neat stuff with frames.
An interesting solution, but all of the people who want world anchors for other legitimate purposes would then counter with "Well why not just disable frames and let us use world anchors for everything else!"
Its certainly not an easy issue. Fortunately it doesn't matter in the slightest right now because there is no 1.3.2 redpower version or bukkit port, and you can always build frame stuff on the existing server, which isn't going away. Or you could make your cool contraptions in the creative world on the new server once they're available, if you just want to show them off and test things.
I'll give it some more thought, but I won't be able to please everyone.