For me not having to PUG is key. I hate PUGs so I have 5 accounts to be able to do my own thing.

However, I also have a nice guild that consists exclusively of RL friends. Often I will play 2 to 4 toons so that we can get a full group. If you have an active enough guild that is tolerant and understanding of MBing, then 3 accounts could be satisfying enough.

I sometimes think about dropping 2 accounts but the lure of occasionally doing 5 on my own is too strong.