Tri-spec - Why not?

As a paladin i've had the painful experience of having to decide between healer DPs and tank.

Healer can't solo
DPS is extremely boring
Tank can't PvP

Now, why can't I have all three? I used to be specced up to 40, which was just fine for PvE (AoE) and couldn't wait to get ret spec at 40 so I could DPS also, however, although it does good damage, it's just FCFS on the cooldowns. So, I respecced the ret to holy @ 65, and now I can't deal any damage in PvP since prot is a joke for single target and non computers.

I propose tri spec should be made available at level 80, costing a large sum like 10,000 gold to make it more attractive for a main character, but not totally out of reach. It's also a good money sink for blizzard, and it means I don't have to shut out a certain portion of the game whilst levelling my paladin.