I went lightweight simply because I got thoroughly sick of lugging a 1x15 cab down 4 flights of stairs and then doing two more trips to load up the car with basses/amp/cable bag. Then there'd be another 2/3 trips at the gig or rehearsal room. Then I'd have to do the whole thing all over again in reverse at the end of the night - it nearly put me off playing for good!
My main reasoning, apart from the stairs/trips issue, was that as most of my gigs are small clubs/pubs with a PA, I really didn't need an enormous rig. I now use a Markbass amp and BFM cabs and I can carry bass, amp, cable bag and cab all at once. Sweet. Admittedly, I think I prefer the sound of my Hartke head to the Markbass....but not enough to warrant the extra weight!
If I was playing the Enormodome on a regular basis and had a team of hulking roadies then my cabs would make Stonehenge feel a bit pathetic and my amp would have more valves than a plumbing wharehouse but I don't, so they aren't.