The most troublesome aspect of many 3rd-person style games is when you're close to a wall or barrier of some sort, the "camera" floating at some distance over your (typically) right shoulder, collides with this obstacle and the provided view becomes, well, ...less than ideal. (Visit games like the original Max Payne, Dead Space or, quite probably the penultimate exhibit, Nocturne, for examples.) In some extreme cases, the camera, now deprived of it's accustomed space behind you, would more or less "wrap around" and provide you with a nice, tight, frontal view of yourself and the corner you're currently standing in. In Killing Floor, I would describe this situation as being tactically unsound.
Some of the Tomb Raider games worked around this situation by having Lara become transparent, and temporarily centering the camera within the mesh of the player's character to a more 1st-person view. I always felt this was the most elegant and practical solution; as soon as LC stepped far enough away from the offending surface, the 3rd-person perspective would seamlessly resume. Other games, such as Fallout3, allow the player to switch between first and third-person perspectives at will, and I'd say in that game, each mode serves a purpose.
Personally, I'm a bit skeptical of how useful 3rd-person would be in KF. Both for the reason noted in my first paragraph, and in the feeling that, even with some modifications, aiming effectively without the benefit of an on-screen aiming point or the use of scopes/iron-sights would almost certainly be quite, shall we say, hit-or-miss.
However, I don't share the notion that having a somewhat more "panoramic" view of one's immediate surroundings that a 3rd-person view might provide would cheapen the gameplay. If felt necessary, it could be hard-coded as verboten in HOE for those who want as steep a hill to climb as possible, as well as being a server-side option in other difficulties. Having to lock oneself into either 1st or 3rd-person perspective for an entire wave would be a deal-breaker for me.
While I doubt I would use it much, if at all, in this sort of game, I still wouldn't object to it's inclusion. Afterall, if 3rd-person is the way someone wants to roll, how is that negatively impacting me?