I personally had the strap come off my bass a couple of times before. Fortunately, I caught it before it hit the floor, and besides it was a cheap-ass bass, so it wouldn't have been a big deal anyway. However, that made me paranoid enuff when I got my first P-bass a few years ago, I put them on it, and have done so ever since. I don't feel comfortable without them . First thing I did when I got my $120 Jazz bass and $550 Strat last year was install Straplocks. Maybe I don't really need them,I don't know, but I feel a lot better knowing they are there. Besides, they're cheap enuff and easy to install.

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'08 Fender Am. Std. Jazz bass, maple fretboard, charcoal frost metallic finish
'10 Squier VM Jazz bass, maple fretboard, natural finish
Ampeg BA115
'09 MIM standard Strat, maple fretboard, sunburst finish
Fender Super Champ XD