There's nothing like terraforming to stir the primate tool-making blood with dreams of glory. But sometimes I step back and I'm a little conflicted, seeing as well some hubris in our grandiose notions. I listen to myself and to you writing off an entire planet as good for nothing more than playing around, and I say "wow!", do we know how to dream big or what?
Not that there isn't a good case for it, not so much writing off Venus, but maybe making the considered judgment that sacrificing one planet that doesn't seem to be too useful as an investment in figuring out how to develop the rest of the Solar System. In any case, even just the thought experiment's useful.
I'm not too sure about the logic of using another planet to build things we wouldn't want to build on Earth. Space seems like a better choice in most cases; there's one heck of a lot more of it than planetary real estate, without the gravity well and the acid rain and the millions of miles away. The question is, what's so unique about Venus that there are some things we can only do there? Or do more cost-effectively than anywhere else?
Maybe it's just scale. We can reproduce intense pressure and high heat in small quantities just about anywhere, but if you need those things in mass quantities, you need a planet or a star. To be honest, nothing comes to mind right away, but maybe it's just a momentary failure of imagination (after all, we've just been talking about reengineering planets, you'd think it'd be a piece of cake to come up with a use for it).
On a smaller scale (relatively), I think Venus would be an appropriate place for more "modest" experiments, like the low-orbit comet. For one thing because it might indeed cause some sort of runaway effect, and well, Venus is used to that kind of thing. It's what got Venus where it is today. We wouldn't so much trash the place as replace one problem for another.
I hope that if we are able to get the space program going in a post-shuttle exploration mode, that questions like this start getting more and more thought and discussion as our potential opportunities expand.
I agree with that. When I come up for air, I see we've all gotten a little cramped in our thinking, horizons limited by the grim vision of endless war and endless problems on Earth, forever and ever. We need something like this to repair our battered psyches. We need to be able to dream again.