At the moment, /nextmap is a vetop+ command. The reason for this is that it's easy to abuse it. I can see two cases where the idea'd be flawed:
- Multiple people requesting different maps
- People knowingly requesting maps with bugs that we've not caught or fixed yet
The first case is likely to be the biggest problem for players, since they'd be wasting 800 points unless requesting it in the last second of the match; a queue would add an extra level of complexity when the random system's fair enough as it is. The second case is likely to be the biggest problem for staff, since catching the people who do this would be difficult without looking at IRC logs, and in the meantime the people would be disrupting other players' games.
There's also "map starvation" to consider. If some maps are particularly popular (CTF-Corridors, Canyon spring to mind) then they'll be voted for often. This would drown out newer maps.
Finally, we already offer this as a prize for competitions - offering it out to everybody cheapens the accomplishment for the people who won it, in my opinion.