When WoW first anounced they were doing Duel spec, I didn't care either way about it, I could understand why they were doing it; the healers and tanks I knew were VERY greatful they were doing it, but I could take it or leave it.
Or so I thought.
A duel spec is a vital part of any MMO at end-game, that is just fact, it's the way end-game works, the way most guilds work.
If you don't want a duel spec, that is fine, nothing stopping you from taking it, but don't try to deprive others from a great game feature.
I've leveled up to 50 as Sith Assassin Tank spec, I spent 99.9% of that time solo, just so you understand I'm not talking about making things easy-mode leveling, duel-spec isn't about making leveling easy, although sometimes it's the by-product.
Keeping a duel spec within the advance class will make your choice at level 10 matter, it won't remove diversity within the game.
So, why do you need duel specs? Well at end-game raiding all guilds want you to have a specialised and focused spec, be it Tanking, DPS or Healing, the good thing about TOR is that you can actually still play to a viable degry while solo in such builds, that sets it apart from other MMO's out there. But guilds need flexibility too, if you have a tank spec and DPS spec, or Heal Spec and DPS spec, when the call comes, you can fill the role your guild demands of you.
If you are serious about raiding, if you are serious about your comitement to your guild and if you are serious about being the very best you can be, you need a duel spec, otherwise you are just short-changing your guild and yourself.
Oh and, if it wasn't for duel specs in WoW, I wouldn't have wanted to become an off-tank for my guild, I wouldn't have looked at the tanking builds and wanted to try it, I was originally DPS, but for helping out my friends my second spec became a tank spec, I got gear for it and tanked my way through heroics for my guild, to help them out and it helped me too, plus I found it fun.