I use Bootstrap when
1. the default Bootstrap theme or a third party theme are sufficient
2. I don't need Foundation's better grid system
3. I'm using something other than Ruby (which for me is most of the time)
1. There's a specific problem that their better grid system solves. I haven't come across this yet, but am aware of it for when/if I do.
2. Client wants a highly customized Ruby-based site with no hint of the Bootstrap look
I like stuff with batteries included. Given bootstrap's momentum, it even has a plethora of design templates as well so you won't have a generic bootstrap looking site.