I've got a SIRT, and really the only real reason is to have a not-real gun that I can dry fire without fear of accident. The laser is kinda neat, but you're better off just learning to read your sights.
FWIW, I break down dry fire into 2 areas - trigger control & target acquisition
For trigger control I do a lot of wall drills, slow fire at small targets, more rapid fire at a single target, etc.
For target acquisition I do draws, transitions, reloads, etc. but I rarely (not never) pull the trigger. I think it's especially helpful to practice things like draws without a trigger pull. I also use a timer for most of my dry fire, some don't
Steve Anderson is another good resource
I haven't shot IDPA in years so this might apply more to USPSA, but I think you'll find that things like reloads and transitions are more heavily emphasized in competition than in more traditional defensive training. I would recommend spending the majority of your time practicing target transitions. I think it's a good rule of thumb that an A zone out to about 10 yards no more than about a yard apart, your transitions should match your splits. I'd try to achieve that and then tailor your practice based on experiences in matches.