To me, Bootstrap is a tool. And I think to really get the most out of it, you need to know the things that go into it, that make it work. And that’s where learning about doing responsive design comes in. I think once you learn how to make a responsive layout from scratch, then you can use a framework like Bootstrap to expedite your production time.
Also it certainly helps to know why something isn’t working from the ground up, rather then encountering a problem and then not having a good background of knowledge to try and debug it. Now I’m not saying knowing responsive design backwards and forwards means you won’t run into problems, but when you do…you’ll be able to diagnosis it quicker.
If you want to get a good grasp on responsive design, check out A List Apart: http://alistapart.com/topic/responsive-design But there’s a ton of material out there to learn this stuff.
Some companies might heavily use Bootstrap, some might use another framework. It really depends on the place. But to me, knowing the meat and potatoes of responsive design gives you a leg up with any framework or just with doing it from scratch.
And going from scratch isn’t terribly hard btw. It’s a case where the more complex your layout is, the more you’re going to have to do to make it responsive and look good on different devices, which is why most of those frameworks were invented, to cut down on having to figure everything out. BUT, that’s half the fun and frustration of coding, it’s working out the kinks, trying stuff, breaking things, fixing them, and learning along the way.
I say go for it. It’s what I did.