I see several problems with this:
- two people picking at the same time, say they both insta-lock the same role
- someone takes an unusual summoner for a role and suddenly no one knows what role they're playing
- I'm first pick but honestly I main mid/adc/jg (could care less which I get). I like being able to pick which of the 3 I take based on what the team needs. This basically punishes me for being first pick (I'd definitely be better off 3rd, and often 4th or 5th would turn out better for the team).
- can't try to organize a team comp (e.g. Someone wants an ad mid so we need an ap top, or our top isn't tanky so we want a tanky jungler).
I'm sure there's more. I've just found in my Draft matches, I very rarely get anyone demanding a role and qq'ing when they don't get it. When last pick demands a role the other 4 tell him he's last pick and he shuts up (and we try to work it out anyways). My biggest problems in champion select are the people who don't say anything. The person who takes a champion that's viable in more than 1 role, doesn't take a role-specific summoner, and doesn't say anything. Or in my case, once I was 4th pick and both mid and adc were left. I asked 5th pick what he wanted and he never replied (I asked like 7 times). I ultimately went tristana (adc) because I figured mid is more popular than adc, then when it was his turn he took vayne with ignite. Like seriously, if you wanted adc I was offering it to you! (Someone ended up dodging even though I offered to go tristana mid then, because frankly with vayne with ignite I think he assumed it was a troll).
In general, I find not talking is the worst. Trolls are going to troll.