Pit Pro adds the following:
1) pretty lights
2) Car ID detection for pit entry and/or exit.
Do you need it? Not especially, no. Some people don't like the pretty lights at all. Also, pit detection is good for only two things: pit lane speed limiting; and restricting refueling to the pit lane.
You need to have a pretty long pit for the speed limit to make much difference, at least until Andy includes an automatic tap of the brakes upon entering the pits. Also, refueling outside the pit lane is pretty obvious if anyone were to do it during a race, so it's not like they could "get away" with it if you didn't have pit sensing. It does prevent accidental refueling, though, which can happen, especially with cars that coast a lot, if people hold the LC button while coasting through a corner. It might be long enough to trigger a refuel in some situations.
Do I have one? Yes.
I like the pretty lights (countdown, fuel level), and I had enough problems with accidental refueling that I like being able to restrict refueling to the pit lane.
In the future, I hope that pit sensing will be more important, such as to track the performance of any pit related penalties, such as a drive-through or stop-n-go. I don't believe SSDC supports this yet, though. PCLapCounter does supoprt Pit-Pro differently than SSDC. I believe they do use it to track penalties as well as fuel.