I can not see how "AP spent in tree" benefits the game, the players, build diversity or balance (there are plenty of choice restrictions and gating without this one). So I'm hoping a Dev will chime in and let us know why they think it's so important?
Here's my perspective:
AP spent in tree is the engine thats driving a number of bad design choices. The biggest being that it's the main reason that live generic class enhancements are pushed into PrE trees. You need to have enough things to spend on in each tree: so you have to stuff the PrE trees with unrelated stuff instead of more intuitive base class "core trees" with PrE's being in their own little "sub tree".
It's an added choice constriction on a system that is going from 3 choice constraints (live) to 7! (SEVEN)
It adds "pathing" as in path of least resistance to get to a desirable ability. AKA: pigeonholing AKA making characters look and feel the same.
It breaks (needlessly) a whole lot of live builds because generic class enhancements are in PrE trees instead of a core enhancement tree.
It necessitates the +20 LR that they're giving us, because we can't recreate live builds with it in place.
It forces them to stuff PrE's with "filler" enhancements that might otherwise have been thrown out.
It's really not obvious why they would bother with it as it forces them to fluff out PrE trees in a way that is non-creative and restrictive for the Developers: In that every time they create a new PrE after the pass they will be forced to fluff it out because there needs to be enough things to spend AP's on in the new tree to meet the gate requirements.
IMO none of this is necessary, it is all needless because the AP spent in tree requirement can be "gated" with the already existing prerequisites... Arrows, total AP spent (as in Live) feat and enhancement's needed, class and character levels, and finally COST. Simple cost...
If something is too good to have without an investment in the tree, simply use arrows.
If something is too good to have at the price you've set, raise it.
If something is too good to have before you've progressed in class/character or AP's available to spend, then put class level or total AP spent restrictions on them.
AP spent in tree is (as far as I can tell) a needless gate that doesn't get you much of any benefit while forcing a bunch of bad design choices.