4 people sitting out to let 1 play is unfair
This is a very, very valid argument and one I fully support. Given the authority, I would never allow a multiboxed character to take the place of an actual person.

I have never multiboxed in a guild raid, because there are plenty of proper bodies to fill the slots.

So if you're marching up to a guild and saying that you want to raid with all 4 shammies or none at all, then you're going to have a problem.

If, on the other hand, you were to apply as a SINGLE shammy and then, one day when they're short on numbers, just happen to mention that you could plug the gap with 2/3/4 if it will help the raid, then you're the good guy instead of the the bad guy.

In other words, perhaps the problem is that you're expecting too much? Get your foot in the door, make your mark, and then float the idea of multiboxing on those occasiosn when there aren't enough people around to create a viable raid.