Another ridiculous thread.
Gain in the Sonar environment is indeed intended to behave as a virtual "Trim" knob to avoid overloading the virtual signal chain (like the PC/FX bin).
The Gain label on MIDI tracks controlling velocity is indeed a little misleading and struck me as odd when first learning the program but aside from the label it is a perfectly functional and useful knob to have on a MIDI track. It takes the concept of Gain and gives it a MIDI equivelent as MIDI is merely data... not audio so velocity is the closest thing to controlling the input. It makes sense. There is NO GAIN IN MIDI!!! ONLY IN AUDIO!!! The audio tracks associated with a MIDI synth all have Gain knobs that control Gain because they are audio tracks.
The fact that the SIT strip uses the Gain knob as a Velocity control seems to me to be Cake using the MIDI strip template as the foundation for the SIT. Since this is a combo track intended to provide convenience to the end user the Velocity function is probably more convenient than Gain which can be controlled in more places and more easily in the chain than Velocity.
As has been stated the ONLY real issue is that it is labeled Gain instead of Vel. It is documented so anyone who has done a precursory look at the documentation knows this is the case for MIDI and SIT tracks.
As has also been stated... if you want that Gain knob to act as a Gain knob you need to move away from the inferior, yet convenient, SIT track format and into the proper MIDI + Audio track format. If you do not want to deal with a slew of audio tracks from a multi output synth then simply use the single stereo output option pretty much all synths have and you'll have essentially the exact same setup as an SIT except with one more track in the Track View but WAY more control over everything. Put them in a folder for quickly removing them from view or use the track manager to hide either one if you do not need to tweak one or the other.
Then you can reroute or load a template for full output of all channels at any point.
This really REALLY is a non issue and nothing to freak out about in such a manner. There are far more pressing matters at hand and frankly I would MUCH prefer that the Gain knob on MIDI and SITs remains a Vel control. If anything is done it should be a simple change of labelling from Gain to Vel and that's it but even then... it isn't really hurting anything and even those who don't read the documentation should be clued in by the Tooltip that pops up and displays velocity info instead of level info in dB when you adjust it.
There seems to be a few posters hell bent on dredging up as many quirks and issues as they can no matter how small. I would be okay with that if they came right out and said "I am on a mission to document all bugs to improve the program" and did it in a productive and respectful manner that helps the software improve and evolve (which is the entire point of the problem reporter and the problem/feature forums). Instead though it's disruptive freak outs, accusations and innuendos day in and day out.
Just describe the problem so people can test it or help you solve it and then, if there is indeed a bug report it through the proper channels. If it affects enough people it will likely get looked at. If not... welp, nothing is ever perfect and we all gotta make decisions about what we are and are not willing to tolerate in life. In this case it is a product for sale that is competing with other similar products for sale that may or may not have the solution to your issues.
A mislabeled MIDI Velocity control does not seem the type of thing that would make me ragequit a perfectly useable product though. Then again... I may be an idiot.