I personally don't see the overriding problem within the current system of not having spec open? If server slots are taken up with afks, meh, it's not the end of the world.
"If it ain't broke, don't fix it" comes to mind here.
But seriously though, I think spectate open would provide a heck of alot of room for exploitation as people have already mentioned with regards to score boosting etc. It's less than a minor inconvenience atm, 20 secs rejoining the game is NOT a big deal. If you have to go do something really quick, what does it matter, go do it and come back, usually afks are only afk for a short time before they leave or come back anyway.
Personally, i think spec should remain locked.