Sample Players/Grain Units — Shift vs Start

tl; dr — Wondering if the Start param (a la Manual Grains/Manual Loop) could be implemented on the Variable speed player. My use case is to modulate the starting position with an aliasing saw, it would work best if this parameter was based on the sample/slice length rather than a fixed amount of time.

Start parameters are relative to the length of the loop with a range of 0.0 - 1.0.
Shift parameters are measured in time, x.x seconds.

I tried to implement something like this with Manual Loop, but when the start position is modulated passed the playback head, it pushes both ahead and gets pretty distorted; a reverse ramp works ok, as it doesn’t push the starting position ahead of the playback head, it pulls it back. If the start position was sampled and applied at the loop point and sync (I think shift is implemented this way), this unit would have worked for my uses.

I can definitely see the merits of both, I’m curious why Shift vs Start is implemented on various Grain and Sample Player units?