It has nothing to do with the game Engine. STO has a huge graphic footprint because they were intentionally going for something of a higher visual quality. WoW can get a way with Phasing because of its low system requirements. If STO had phasing only the high-end machines would be able to play it. Everyone else would just lag-out, specially when their computer system was tryign to preload the next zone. For the game to be viable for lower-end systems some compromises needed to be made.