When politely asked, I have always allowed other bass players to use my rig - never had a problem. Only once have I refused. My band (London based) had a gig in Brighton. Rather than everyone driving their equipment, we asked the other band if we could use their amps and drums - they flatly refused, so we loaded our cars and brought everything. The local band then swaned in late, no back line with them because they couldn't be bothered to bring it and said they were using our kit. We always prided ourselves on being polite and friendly to bands we shared the stage with, but took great pleasure informing this bunch they wouldn't be playing unless they went home and got their kit, because they sure as well weren't using ours.