Here's the thing that neither side is talking about.
For the "Talent trees are good, see!??!?!" side: Yes, the promote some diversity in what you CAN build for, but there will always be some kind of optimal build that you will follow a guide for.
THAT BEING SAID
For the people who argue it doesn't make sense to make this change because there will always be an optimal build: Who cares? It doesn't affect you in any way. You are currently probably raiding with an optimal build anyway. They already mentioned that there will be nodes on the talent trees that function in a similar manner to the way that the current ones do. You can choose between one of three options from said node. You might not be changing your talent trees other than those nodes potentially, but then you're just playing the game the exact same way you are today. No real difference.
Either way, the main thing I see coming from this system are as follows:
Lower levels will be more interesting in general. The reason I have more fun leveling in Classic WoW from 1 to 60 is because I'm constantly shown the carrot on the stick for that next level. You're shown the talent trees and you can very easily see what is ahead of you. Same thing with the way that learning spells used to work. You'd have to go to your class trainer to buy the new spells, and then you'd be able to see the next spells you were going to get in the next few levels. The way leveling currently works in WoW, you kinda just flash gold and then a spell appears on your bar. It's a pain to navigate through the spellbook in order to see what spells you already have and what spells you don't have yet, so you're not really excited to see what comes next.
It's just a very different feeling between the two games, and I think the talent trees will succeed in bringing back that excitement that leveling has. Is that useful for end game? No, but it also doesn't affect end game. Not every single thing in the entire game needs to be designed for you and specifically you to enjoy. Leveling is a part of the game, and some people enjoy or had enjoyed it in the past. If something is added that doesn't negatively impact you directly but positively impacts somebody else, then it's perfectly okay.
The other thing that's positive is that, while it might ultimately end up with cookie cutter builds as far as the eye can see, it doesn't HAVE to be that way any longer. The talent options you've been given since MoP are not really build or playstyle defining for the most part. There are some exceptions to this, but generally every single spec is going to play like every other spec because it was hand crafted by Blizzard to do so. Even if it doesn't work out, there's a CHANCE that it might create more diversity in how you build your spec. As stated before, if it fails in this goal then all that happens is that you just play the game EXACTLY as you do today. There's no "oh shit, this didn't work and now it's negatively affecting the way I played the game!" It either brings diversity into the game, or you just cookie cutter spec like you do today and ignore talents the same as you ignore how your class and spec is built today.
There's really no negatives to this system being added so might as well add it for the few positives that it may or may not create.