Can't see the flat dwelling 20 somethings being desperate to spend their spare time up to their elbows in manure.
I'm a flat dwelling 20 something and trying to find a house with a garden so I can, and several of my 20 something flat dwelling friends are the same.
I'm not bothered about allotments personally, I'd never use one anyway, but I think it's a good idea to promote such things, especially within the community who can't afford much on their benefits and who otherwise eat a pile of junk. It's hardly a huge dent in the public coffers for quite a few benefits for several generations. But I don't expect everyone to see past the "I want some land to grow for fun" shortsightedness.
If you're a tax payer, you really don't want to encourage the Council to go down the contaminated land route on this, it's massively expensive just finding out what's wrong (if anything) with the land, let alone getting it fixed.
We're talking relative costs here, and buying prime-development land and providing reasonable bus services out of town comes in significantly higher I suspect. While I'm not in the business of altering the land, I'm aware of someone who is and who does this day in day out as their bread and butter, and while it adds a percentage, it's not a massive percentage from what I hear.