I think this was done to curtail some of the OP builds from OS1. If you had enough AP, a single character with enough skills could flatten a smaller group of enemies, no sweat. With the set AP as now, they can also remove some bloat in AP costs. Instead of small scale abilities like Rain that don't really attack but can affect the battlefield costing 4-6 AP, they can now flat cost 1 for everyone. It opens up build paths, and, with gear that has +1 to different ability school, allows you to more easily use abilities in that school without being penalized. It also adds value to haste as a skill rather than just dumping points in a Speed stat and not needing such a spell.
