People who demand their ideas be implemented into your mod are annoying as portugal but easy to ignore, people who demand their ideas be implemented into a commercial game may well represent your customers, so even if they're annoying as portugal they can be pretty useful. In terms of ideas guys who work within the team, their usefulness rather depends on what you need. A mod or game which prides itself on interesting story and plot may have space in its team for people who's sole role is to be an ideas guy, otherwise probably not.
Personally I would like to have time to make an M&B mod, but I don't (and I don't even have a computer that can play m&b effectively) but I still sometimes think about ideas for it.