The problem with merit or tree-based allocation is that, by design, you're locked in to a rigid build you can't easily make changes to, so you're really just stuck with what's considered "the best" build possible (or at best one of 3 of the best builds).

To me, that's the beauty of a well crafted point-allocation system. You can strategize according to the situation you're about to face and place points differently according to what you consider important at the time. If a new situation arises, it's simple to make an adjustment.

You wouldn't walk around in a fire-resistant build everywhere you went, but going up against Ifrit, you might throw one together if it wasn't too inconvenient. A mage might not normally throw a lot points into Vitality for more HP, but if they know they're going to be eating a lot of big AoEs, they may consider it for survivability. Or maybe not. But at least the option exists in a way you don't get with merit or tree-based allocation.

It gives you choices for the moment and opens a discussion about what works in a particular situation, not what works best in most situations, so build this one way and never think on it again.