At one point, it was the intent of the FAA to eliminate "ARSA's", and they nearly did, when the "new" alphabet airspace came into being. But, then they realized that they really did need something in between Charlie airspace and Delta, when there is a radar approach control facility, and a lot of traffic. Thence, the TRSA, and they've been adding them ever since.
Participation is indeed optional, HOWEVER, if you contact Approach inbound, or contact Clearance Delivery outbound, and you do NOT want to participate in TRSA Services, you need to tell them on the first call up: "Negative TRSA Services". At that point, you are dealing with Delta airspace, pure and simple. You can also opt out at any point by simply saying "Cancel TRSA Services".
So, what does the TRSA do for us, if we opt in? When you opt in, the controller is REQUIRED to provide POSITIVE RADAR separation between your aircraft and other PARTICIPATING TRAFFIC. There are several things that meet the criteria of positive radar separation, including green between on the radar, 5 miles (I believe) between, and altitude separation with Mode C. OR, if the controller calls traffic for you, you tell the controller you SEE the traffic, the controller will tell you to maintain VISUAL separation from that traffic. At that point, the separation issue is YOUR baby, not the controller's problem, though they obviously won't ignore you.
The only glitch is that participation is optional. So if you participate, there may be aircraft other tehre without a transponder, and ATC may not be able to see them and separate you...... I've used TRSA a LOT (at FAI and FAR) and generally, the controllers work pretty hard to keep traffic separated and flowing.
The TRSA at FAI came about because the FAA wanted CHarlie airspace, and proposed it. A group of pilots there asked them to justify it, based on traffic, and FAI only met one of the criteria for Charlie airspace. The issue was ATC staffing, which is in part based on traffic count. We proposed TRSA, since participating traffic counts in traffic count, which they don't in Delta airspace (if they're VFR).. ATC agreed, and has worked beautifully, got them the staffing they needed at busy times, and works for the pilots. No Mode C required.
MTV