pot does play a very unique roll interacting with other drugs. I'm a daily pot smoker and a weekly tripper. A few months ago I quit smoking pot for about 10 days, when I took some LSD. I took 4 or 5 hits of decent acid and for some reason I was unable to trip. Mild body tension, little hints of peripheral visuals but no legit mind warping. I decided that I had to smoke pot to save my trip, so I loaded up a bong and got a N2O balloon ready to exaggerate the rush. Took the biggest bong rip I could followed by several deep breaths of the gas and my mind was catapulted into the center of a vibrating oscillating blindingly bright universe as the pot, N2O and all the LSD kicked in at once. After that the LSD worked full force all night. So for some reason I can't trip without pot.
Another time after i quit pot for a month, I took 4.5 mg of DOC and smoked pot for the first time. It made the DOC trip into one of the most insane and powerful trips of my life, and kept it going full force for over 24 hours!
Another thing that should be considered is that pot is very good at triggering "flashbacks" or exaggerating residual effects of recent psychedelic use. If I smoke good pot I can usually re-create the visuals from my last trip, whatever substance it may be, sometimes even many days later. Or for example my brother had a nightmare trip that traumatized him alot, and whenever he smokes pt even years later now, he gets the same feelings and psychological effects that he had from the trip.
I wonder if pot use preventing dreams has anything to do with this. canabanoids sure are unique!