Hi Ho Kevin: Using the existing saw fence is a mixed bag. You obviously have to add an addition to the fence somewhat like the sacrificial fence that you use for rabbeting. My table saw fence is larger than I would need for routing. It also maintains itself parallel to the saw blade which is not needed (and often not wanted) with a router. It is convenient to simply loosen one end and adjust the router fence closer or further from the blade.
Of course the advantage is that the saw fence is already there. If I wanted to use it I would build a plywood fence that has the features that I want and slips over the Bieselmeier type fence on my saw. Remember the fence must bury the largest bit you have (or will have) without hitting saw fence so the router fence addition would be spaced a distance from the saw fence. You would have to pay close attention that the fence can never contact the router bit. (Look at all the router fences that get a lot of use and notice that most have ding marks, especially the two-piece fences)
Anyway, I'm sure you will get other opinions and good advice here.