If it's not useful, then we can make it stronger. Yes, they can't make fortresses everywhere... buta lso such a big battle wasn't everywhere.
For me the building summon is a great tool to show the character of the dwarves and have a unique spell. I agree that it's a bit tricky to place, but I wouldn't delete this spell for a normal summon. I have written many reasons against it already. The only real reason against the building I've read here is "It's difficult to place and to weak".
The first one is a bit tricky to change, but I'm trying to change the maps so that there are always some palces for the fortress and for the second one... well, it would be easy to make it op.