I like having 2, even with specs that have 3 unique positions. The barriers for wanting more are small ( 30 gold and an action bar set up, 5 mins with practice). Having three specs however sets up one of those negative feed back problems that wow faces
" we need you to play healer, this is one tank fight"
" but I don't like healing, I wanted to tank and some Dps"
" there is trispec you have to"
I compare it to the valor upgrade system. At first people loved the idea, but soon an non upgraded piece was looked down on and inferior. You were required to fully upgrade to be good, it was no more a bonus. You now are obligated to cap every week.
I imagine the same thing would happen for tri ( and up spec). A paladin would become obligated to build Rey and tank and healer because it's expected, even if they hate one of those roles. And worse they would probably be expected to be good and do all three or be viewed as one if those "baddie dual spec palies"