@Christoph Hart:

Do you need just a constant offset or do you want continous control?

Well, I'm guessing with continuous control, it might be possible to apply an LFO to the phase which may make for more interesting sound design, especially if you have the ability to reverse the polarity of the waveforms. For example, it's possible to make PWM with two saws, but that would only be useful if the synth you're working with doesn't have PWM to begin with like Image Line's Sytrus.

So that you could have more freedom with sound design and stuff. Same with unison. If all the oscillators' phases start from the beginning, it doesn't really work for thick pads or supersaws, etc.