For example, six points in the Elementalist tree gives a fledgling mage a tanking pet, a damage shield, and an ability that basically removes out of combat mana regen downtime by draining your charge bar (built up as you cast spells, sort of like a Warrior's rage bar in other games). You can stick these types of capabilities into your character like building blocks, ensuring that you have the basics that you need to play before advancing into a tree that you really wanted to focus on.
The results can be remarkable. I was not so impressed with the caster Clerics initially. Then I respecced and added some points in the Justicar soul. Justicars get low levels of self healing to begin with, but investing 10 points enhances that baseline self-healing to significant levels. Suddenly, my caster priest was noticeably more durable than any caster I've ever played before, combining medium-heavy armor with significant amounts of self-healing just from my DPS spells.
Rift is not the only game to offer customization options. Building a multi-class DDO character feels slightly similar - for example, many players start their characters with a level of rogue to pick up some sneak attack damage, the "use magic device" skill for self-healing, and some other goodies. However, respecing that DDO character later is rare or costly (in the cash shop). Rift characters can pick up their second role basically immediately, and my cleric has her third slot (out of four) by the early 20's. Respecs are completely free until level 14, and seem cheap thereafter (with the cost scaling as you level, a sensible plan that still wasn't in WoW when last I checked).
All that said, I am leveling the mage now in part because I want to see whether it even makes sense to do so. In addition to having the most healing options in the game, my Cleric can melee, cast DPS spells, and even off-tank. By contrast, the mage gets only a single healing soul (which may or may not be the best choice for certain purposes) and is compensated with a variety of ways to nuke and buff/debuff. Does one character really need half a dozen different variations on DPS casting, especially when many will include a common core of skills (like the six points in Elementalist)? I don't know the answer, but I'm working on it.