Not having player collision with your team mates is something I’m still on the fence about, but not having it with enemies causes a lot of weird situations (especially when melee-ing) where you phase through them and lose track of them. Worse yet, sometimes they’ll phase through you after you melee but not kill them, and then they’ll melee “behind” you and it’ll count as an assassination so it insta-kills. Here are some examples, really showing why player collision should be enabled against enemies only.
(Can’t post whole links so I just posted the end url for youtube videos. Just copy these after the normal youtube url in your browser, or you can google search the string and the video should pop up.)
As for why it might want to be considered for allies as well, there are situations like this that just make the exclusion of it feel odd.