I actually strongly disagree: You need to be a user of libraries not a developer of them when you work in business (that does something other than sell libraries).
Most businesses you want to work for will also develop libraries/frameworks that are used in house. Or they might develop applications which consist of more than just plugging libraries together.
You will develop code you reuse, you may even pack it into libraries, however you shouldn't be thinking this is a fundamental process of the business. It's a side effect that makes some things simpler when you do similar things repeatedly.
I agree for most part. I think a good web developer is one who can use the existing libraries efficiently to get the job done, not necessarily build one. It's hard to build something when you have to get the job done.
Most businesses you want to work for will also develop libraries/frameworks that are used in house. Or they might develop applications which consist of more than just plugging libraries together.