I can think of a couple of reasons for the exemplar system that explain why they did it that way. The biggest is the eventual (hopefully) ability to do tfs and trials that were missed by high level players. If you got to keep all you powers and slots, it would unbalance the early tfs.
I am guessing that the reason you make no real advancement was to keep people from abusing the system. It is not the best answer, but it was probably the easiest/fastest.
As for a reason to use it now, you exemplar down to exactly your partner's level. Then you both have comparable abilities. SKing brings someone with less powers and slots to close to your level. If you are a high level playing with a low level friend, how much help is that person going to be with only a few powers slotted with a couple of maybe DOs? This way both parties can feel useful. Not very good for a power gamer viewpoint, but nice from a fun/rp viewpoint.
