Case in point: we were doing naxx 10 for achievements, and I was boxing 4 dps. For 4 horsemen we had to design a strategy around me boxing, rather than just splitting up the dps... We ended up having me and the other dps go around and work each boss slowly. The raid had to accommodate me. If you have a similar situation, more power to you. A standard guild won't want to make exceptions, unless they really need to fill spits, and/or hate to pug.