The biggest reason for me to deny it is the "if you give a mouse a cookie" scenario.
If you give the players 2 specs, they will want 3. If you give them 3, then they want a 4th tree for all classes. If you give them a 4th tree then they want 4 specs.
It continues until you reach either a point of total lunacy (10 specs/trees anyone?) or you give up and say "no more specs, just have everything baseline."
For those saying that 3 is all we'd ever want, I passed by a "4th spec ideas" thread on my way in here. It's already happening.