I haven't been on Beta, but how long is a full-power Demon Form lasting these days? I can't imagine that it goes over a minute or so if you're actually casting flat-out, so not being able to cast Bane of Doom (as opposed to the semi-Doom via the glyph) in-form is more or less a moot point.
Speaking of the Corruption-Doom, we BADLY need to see what its tick rate is going to be like and if it will generate Demonic Fury (and if so, how much). That glyph could become an instant necessity to free up time (not to mention the scale factors could be incredible, as posted on the front page it will scale with fully four times your spell power). Not to mention it would make that Corruption cast go, again, for a full minute. Enough time to pop the DoTs off, hit your Cooldowns and go Demon, only to finish up well before you need to worry about refreshing.
EDIT: forget what I said about scale factors, after some digging in WoWDB I found that this works out to be identical with Corruption over the course of a minute (Corruption is 120% over 18 seconds). Still might be worthwhile for saving you a GCD every 20 seconds or so, if the DF generation rates are the same or at least close.