Hi Joel,
Thats correct- SinOsc is just a naive implementation of a sine wave. I don't know that there is a reason for this other than simplicity.
In performance-sensitive situations I have recommended using either MagicSine (if it is not necessary to change the freq/phase rapidly) or Gen9/Gen10, which are sinusoid wavetables.
I have been interested in developing a chugin that is just a really fast/mathematically accurate wavetable sine. If you'd be interested in making something like that, or collaborating on it, that'd be very cool. If we got that in good shape we could replace the default sine, if there is no other reason to keep it the way it is.
spencer