I'd like to see live, as in on the map, not instanced, guild housing created. It'd have to be extremely expensive, have a very limited number of marked lots available per zone and in traditionally non-combat regions, for instance, not outside a cave Orges or Yeti are camping in but a field with just a few critters running around would be fine. Throw us a free second stone to be able to port in and out and you could, possibly, bring the community closer together. And absolutely, on PvP servers bring on the destructible housing. Yes, it could possibly lead to griefing but when was the last time open world PvP in WoW wasn't an 85 beating down a bunch of level 40 guards and level 30 characters. Unfortunately I think the way WoW is set up same side Guild versus Guild combat would be difficult, at best, to implement. But there does need to be a way to take control of a property if you are Horde and another Horde guild currently controls it.
As for how useful it would be, that depends on the community. Granted this was before General and Trade chat, but several guilds got together in UO and built a tower as a safe haven for players to meet up, set up raids, talk, etc. This would still be a possibility today, and one I'd welcome. I can't tell you what half my guilds characters actually look like even though I talk to them daily. And my time at the YMCA in Yew is still one of my favorite memories from early MMOs. I doubt it can be replicated but it wouldn't be bad to try.