I don't think you're forced to do it, you choose to do it because it gives you an advantage in game. I find using sturmpioneers right next to the building chews up whatever is garrisoning it pretty quickly, but I'm only talking about the first few engagements of the match not all game cause that would leave sturms pretty vulnerable to squad wiping vehicles and upgraded infantry.
It was not in this replay, but against a maxim that wouldn't work and against Riflemen you run in danger to have a squad wipe. Especially on maps like Semoski this can be really hard to deal with, since the LeIG takes far too long to clear any building.