I've been opposed to XOR drawing for two reasons.
One, no one asking for it seems to understand what it would actually do. It doesn't simply cut shapes out, and it wouldn't erase upper layers. People say they want it for LttP-style dark rooms, and it simply isn't appropriate for that.
Two, Allegro's implementation is problematic. It doesn't XOR the colors; it XORs the palette indices. That makes it too unpredictable to be useful at all, and it would be very difficult to support in the future.
I would like to see additive and subtractive drawing modes, but adding those in 8-bit color would mean keeping two more translucency tables. I don't think it's worth the added effort and overhead.