1) Git log becomes useless.
2) You're merging code via a UI, without ANY granularity whatsoever.
3) Does it not just FEEL dirty doing this in a UI?
4) A little appeal to authority: http://www.wired.com/2012/05/torvalds_github/
5) and a bit more: https://github.com/torvalds/linux/pull/17#issuecomment-56637...
6) Efficiency lost w regards to minor changes. Youd actually write a comment to have someone change something and then RE-issue the pull request? Sure you would, because youre middle management.
Pull requests seem to be a hacky way for management to get involved with something they understand at the loss of control in arguably the most complex phase of coding: INTEGRATION.