This is exactly what makes the game so fun. Unlike WoW, you can mix and match Souls, allowing you to tailor the character to your play style. You can even try some really unruly combinations, just because.
For example, in WoW, if you roll a Rogue, you're a rogue. You get stealth, poisons, lots of melee damage, and stuns. On the other hand, if you roll a hunter, you're a hunter. Sure, you can specialize in traps, or specialize in pets, or specialize in ranged damage, but in the end, you're just like every other hunter out there.
Now, in Rift, I can decide that I like ranged combat, but I hate pets and I also enjoy being stealthed. So I make a Rogue, and for my souls go with Marksman, Nightblade, and Saboteur. On the other hand, maybe I like to fight up close, but I also really wanted to have a pet, so I go with Ranger, Riftstalker, and Blade Dancer. I could even choose to be support / healer, but also throw bombs and set up traps by going with Bard, Riftstalker, and Saboteur.
The point is, there's no right or wrong combination. Yes, some souls naturally work better with other certain ones, but every single soul has abilities that will be a benefit to the class, and thinking outside the box can really lead to some excellent game play. That, and the fact that you can purchase extra roles (templates, which can be switched to on the fly) means that experimentation is going to be a big part of this game. Want to do Bard, Riftstalker, Marksman? Why not, go for it! This also means that you never know what to expect from your opponent(s). Just because you see someone shooting arrows with a pet at their side doesn't necessarily mean you have to watch out for traps at their feet. For all you know, you could close on the person only to find out that they're actually very potent in melee combat, and the fact that they whittled you down to 2/3rds health at range means that they can easily finish you off hand to hand.
So far I'm very much enjoying this game, but for me, it's going to come down to the end game. If the game ends up being nothing but raiding, battlegrounds, and collecting mats for crafting or rep for gear, it'll just be another WoW. However, if they manage to weave in something else than a constant grind for better gear, I'll be happy to pay them each month for the joy of playing.