My opinion in that is players want to play a level to have fun, as I've only recently realised. While I also encourage usage of the entire level so people have to move around, in most cases, people want things for convenience sake. For example, EBs at the opposite side of the level to a powerup in a wall... You'd go for the EBs, come back, and pow, someone's taken the powerup. Weak example yes, but this goes for most scenarios... In a 16 player situation, imagine taking a powerup and not being able to make kills with it because you're in a section of the level where the powerup is rendered useless? In the time you take to go to the other section of the level, you can die on the way making you pretty much waste the powerup and life. This will *discourage* people from going for that powerup in future. People tend to have more fun if they get the powerup and overpower people with the sheer force of double damage, and that's how it should be.

As for putting useless powerups, I'd only use that as a way to provide ammo, else I'd never encourage it, especially if you're trying to make it a section of the level you want people to go to.