Recent versions of Hammer - since L4D2, I think - have an Instancing function where you make an arch or whatever, have as many instances of it as you want at whatever locations/rotations, and when you modify one you modify them all. But I’ve never used it, and relying on it would only work for those more recent versions of Hammer and take other editors off the table, which would be bad.
I agree that that would be a seriously awesome feature if it could be done well.