I think it can be down to habit, it can be down to friendships, it can be convieniant, it can be a number of reasons. My last band had a pretty woeful second guitarist but he was great as he did what he was told, was a great platform to underpin solos and as importantly, he always showed up on time with money for rehearsal.
I'd rather work with a steady musician who was a good laugh and reliable than a virtuso primadonna. However I don't like people who can't play in time. Poor gear is never an issue and stuff can always be bought once the band is earning.