SWTOR tried back when it was sub based and before they streamlined the skill trees. It ended up the same as we've been saying. One spec towered over all the rest when it came to group based play. Even worse, once they retooled the trees to eliminate the overpowered builds, they inadvertently nerfed whole classes. Commando used to be an amazing turret DPS/healer hybrid. They nerfed both trees and no one wanted to raid with a Commando until they forced people into only one tree. Now, Commando is a great AoE healer and decent turret. Scoundrel is another. Due to Rogue classes reliance on stun locks to get their main dmg off, Scrapper Scoundrels were gutted to the point of uselessness. When people began to spec hybrid DoT/HoT specs, they nerfed the healing tree. Now, after the streamlining, Scoundrels are great burst healers and sub par rogues unless specced into Dirty Fighting.

Allowing people to hybridize only opens up a whole can of worms for class balance. Either it's amazing to the point it's the only option or it's sub par to the point no one will bother taking them. You make them all even and you homogenize the game.