You mention "being that useful" but you never state in your post what your end goal is. To wit, I made a decision I'd have 2 multiboxing teams, one dedicated/geared for PvE heroics, and one dedicated/geared for PvP battlegrounds (not much into the Arenas). Because I am getting that specific, I can tell various things:

PvE Heroics means I need 5. It also means I need all 5 at 70 so best idea is to level them up together. If you aren't one-manning heroics, then if you don't want to to do 5, don't do 5. That easy.

PvP can be done with anything, 2, 3, 4, or 5. 5 is sexy but really not necessary. 2 is incredibly easy to level to 70 in no time flat and DOUBLES every precious tick of Unstable Affliction and Corruption my locks do. I went conservative with it because I can do collection quests without ripping my eyelids off.

It ALL depends on what your goal is at 70. Define THAT and we can almost tell you exactly what setup will work best.