You may or may not know that in the first round of every match, players can move a second or two before the counter officially reaches zero and the bell goes off. The devs have apparently said that this isn't a bug, despite it being a drag on the experience for some of us (like you and I).
As far as i-frames are concerned though, I'm gunning for anyone and everyone as soon as I can, so the argument against i-frames is this: why should I be put into a vulnerable position for going on the offensive as soon as possible?
Being able to move before the bell goes off still sucks though. It may not be a bug, but it plays like one in my opinion.