Comment 1 for bug 1342119

Hm, well the reason we didn't implement that (yet) is that DPMS support brings in an extra dependency and you can never know whether the hardware supports it.

Right now we do screenblanking only because that was less problematic (no extra dependency, no support problems).
I agree though that it is desirable to implement that (and it shouldn't be too difficult), so I guess we'll take care of that in one of the next releases.