... would it not make sense to use *any* runes (even DD) on OB whenever you can remain GCD locked while doing so?
I think that depends entirely upon whether or not you're capping runes - a situation in which you literally can't spend your runes faster than they seem to be refreshing. Definitely happens to me during bloodlust at my current haste level, at which point I do indeed burn runes as fast as possible (i.e. Obliterate), no matter what the combination, in order to maximize RP gen and minimize rune-capping.
However, if you're remaining GCD locked for extended periods of time AND burning runes with Oblit constantly, then you've got a haste problem (you're too hasty).