I'm personally not against skill trees, but the devs would have to very carefully implement it. The best way to do skill trees in my opinion, would be to balance them in such a way where the choices are legitimately competitive with one another and that there is no "best" or optimal way.

Now I've seen the sentiment thrown around of "who cares what's optimal?" and I certainly appreciate the thought, but what happens? Eventually some theorycrafter does the math and figures out what's optimal in a raid/group scenario and soon people begin to post about it. Someone asks in the game, "what should I spec?" and they are told to look it up online and they find the evidence of the optimal spec in that same thread.

Soon when someone asks them what's optimal they tell them what to spec and soon the whole thing snowballs until it reaches the point where everyone (whether they group or not) must be that spec or they are considered a bad player for not knowing how to spec their own class. This happens in every game and those in higher end group content will often do whatever it takes to max out their dps/hps even for something as paltry as a theoretical 1% gain.

So how can we do customization without that happening? I have to go by the suggestion of someone in another thread (I can't remember who you are but I applaud your idea) about making the customization purely aesthetic. Using BLM as an example, give them a talent tree to change the color of their fire, no changes to mechanics but let them have a choice of blue fire, purple fire, or green fire. That way there is customization but by keeping it purely aesthetic, the debate about "optimal" won't work.