This is based on my experience doing an initial at FSI and having one of their SICs for my whole training footprint.....SICs are there to simply play the SIC role when FSI has a client going through a type training and there are an odd number of people in the training class or in my case when you're the only one. They are there to perform the necessary duties running checklists, slinging gear, etc while you learn to fly the plane and complete the training program. One thing that is nice is that flying the sim is all the SICs usually do. They know the instructors and their training profiles and can be a great help when bells and whistles start going off.
Based on chatting with mine, he indicated that he stayed on one fleet type at one location and they had enough work for him to be full-time. That's not to say they may send you TDY at another training facility. He was young 20's and with about 700TT. They certainly do get to meet a lot of different people and network which should work in their favor in finding future work.
Hope this helps...