The short stroke on these shocks is one of the reasons they are so crucial. The shorter the stroke on a shock in general the more spot on the valving/spring combination needs to be because it has so little distance to do it's job. The harder you push your sled in rough terrain, the more you'll notice the shortcomings of a stock FTS. I'm a big guy and like to push my sled pretty hard getting to the good stuff so my track shocks get a workout. The FTS probably takes the most beating and is the first one I'd want a reservoir on. The small oil volume is getting overused most in my case. I'd say the primary reason you see RTS with reservoirs is because people want a compression adjuster on that one the most. It's the second most important one to have a reservoir IMO.
Order of importance in upgrading shocks on my sled are RTS, FTS, and then the front pair. Order of importance for upgrading springs is front pair (for being on one ski), RTS, FTS. Now think about that!