A little input here from someone who is working with no external instruments except my drum brain, which happens to be a Pearl Mimic Pro at the moment (coming from Rolad TD-20). I never found Instrument Definitions to be terribly useful, but only really ever had one "external" synth, which was ironically on a card inside the computer and used an editor for it, which was WAY better. In that case, it was XGEdit for an old SW1000XG card and it worked so much better than any of the instrument definitions or Sonar control panels (can't even remember what they were called, but we ugly and clunky at best).
All that said, I will truly miss the Cakewalk drum maps. I used them for every recording for the midi data I recorded from my Drums. In Studio One (which I am adapting to), there are maps to a degree - you can map the notes to names and a few basic things, but the parts I see most missing are:
Notes that have no "length". Drum notes don't have duration and don't need to be shown as such.
Velocity tags ON the note instead of just in the controller pane at the bottom are very useful for quick and accurate adjustments.
BIG: Limiting the notes shown in the midi view to ONLY those that are mapped is a HUGE help in looking at a drum track rather than having to see all of the in between notes that will never be used/recorded.
I didn't really use the IN to OUT mapping to change one note to another to control different sample libraries or instruments, though I'm sure that those were useful to others.
I will try to put in a FR over at Presonus, but it doesn't seem likely that I'll get a lot of ground there since there is an overwhelming amount of Sonar Whining over there as it is and I've had issues even getting Sonar folks to adopt the Drum Maps. I even created tutorial videos on how to use the feature, how to set it up, etc, but lots of people still stuck to the old Split Notes to Tracks CAL, which is far inferior in my opinion.