there doesn't seem to be much of a point to coding a .SPC tracker given XM and MOD trackers
I had the same standpoint before I actually get into SNES development. Now I think that a SPC tracker would actually make sense. The main reason is to allow you to hear exactly what you get, because difference between what you hear in a XM tracker and in resulting SPC is major (not only in sound quality, but also in envelope timings, volume levels etc). Other reasons are that you simply can't make good use of the echo feature while composing, samples are filtered and looped in a special way that could affect to the timbre greatly (and you have to resample them to a very special frequency 33488 hz to make them loopable), you probably would like to use hardware ADSR, which is difficult to control using a converter, also control on the memory usage is difficult, especially if you going to use echo.
SNES does have unique tone, that's true, because it is not just samples, it is special kind of samples.