One of the major problems with job customization is that it ends up as an exclusive practice rather than an inclusive one. It's already pretty bad in that many groups won't touch a WHM or SCH that doesn't have swiftcast, or a WAR that doesn't have provoke. can you imagine how bad it would be if everyone had access to most of the game's abilities? Being an effective tank would mean you have both tank classes at level 50, as well as PGL for all the abilities it offers.
It would basically lead to no one being unique, because there will ALWAYS be a "golden" way to do things.
The abilities as they stand simply don't have enough varied applications to warrant the level of customization you want.