It would be fun, but destroy any semblance of pvp balance. Asherons Call basically did this, with a few twists (training vs specialization, player determines where to spend primary stat points), and it was a lot of fun.
"Look around you. We're all liars here, and every one of us is better than you" - PB
"I see we have visitors. Two arms!" - Mar'tak
This isn't Pokemon: Class type combinations would not be fun in WoW.