My typical staff:
11 pitchers: 5 SPs, 1 LRA, 1 Mop, 4 SU.
No designated closer, for exactly the reason that Mike describes above.
If one of my SU guys is noticeably weaker than the others, then I go with 3 SUA with the weaker guy being a SUB; otherwise, I have 4 SUA.
I'll try to skip my SP5 if the schedule allows, so my rotation might typically be a 41/2 man rotation. Sometimes when I have a long stretch of games with no off days and start to see fatigue in my SPs, I might have to give my LRA a spot start, so I swing him into the rotation and move my Mop guy to LRA for that game.