I've run into issues where it wouldn't finish the logon because of /nick popping up. I agree that it is annoying when that happens and you're not there. If anything, I'd say to pop that up and wait X seconds and if nothing is typed in, THEN use a random number or _'s just so that you are able to automatically get back on while still allowing it to halt. I do see valid reasons for wanting that to halt logon.
As far as being banned, I know that on Undernet, I set up our bot to automatically ban any nick with letters followed by 2 numbers. It does allow them to unban themselves and get on an exception list if they are real people, but it still bans them the first time they enter. The reason is because many bots on undernet come in with nicks like sara21 or laura25 or lisa18 or whatever (female name + age) and spam everyone with porn sites. This became bad enough that I had to add that code just to keep them out. I don't think it's as bad now, but real people aren't too badly affected by it and they are told how to remove the ban and get on an exceptions list, so it's easier just to leave it on. It may be that other channels and/or networks have also set up bans for the same kind of reasons.
I think either doing the short pause before changing the nick, or using _'s, or allow users to set up more than 2 nicks to use so they can add maybe 5 or even 10 different alternate nicks that way it's controlled by them.