I carry a tripod and a monopod in the vehicle, at all times. The stick is in there because I sometimes need a walking stick and the monopod collapses to a convenient size.
I've found that I use the monopod much more often than the tripod. Partly because I have it with me.
I'd like to get some feedback on your feelings on this. Is the monopod sufficient for most occasions or not usable for most occasions? Is your opinion based on the type of shooting you do or on having tried and decided?