A surprising amount of players have no problems with a DTS only track though.
Trailers extras whatever. Extra discs for god sake. If you can't afford $.30 for a second blank... Frankly though I don't see where the X0 project should even waste their time with them.
As far as space on a DVD goes, sure DVD standard can only do a max of about 10mbps (divided between all video, audio and subpicture streams), a large number of 2-disc Special Edition-type releases still only put about 5-6 mb of space towards their movie-only disc.
I don't know if they're lazy or if it's just cheaper somehow to produce. Ever see a CBR 9mbps 2 hr movie only disc? There's space for it on a DVD9.