Embedded software has an additional dimension as compared with the system programs: the PCB. It is not an accident that I like to have the PCB design software on the very same machine where I do the coding. The program and the layout of the PCB are tightly bound together and when done in a good, interactive way the result is simpler, cheaper and easier to manufacture.

With commercial libraries one cannot easily change the in/out pins or even the target MCU. Ofcourse there are libraries that are supplied with complete source code but then it is merely a matter of taste whether You code it from snippets or butcher the library.

COTS (Commercial Off The Self) libraries are perfectly fine when used to solve tasks that are not layout dependent like USB, TCP/IP and propably a thousand more things. I would not code a library for USB - the protocol itself is a nightmare not to talk about 1-wire ...

There is one thing I always do with the COTS libraries; I build an isolation layer between those and the inhouse code. It is not too uncommon that the device they support disappears and You have to replace something. In this case You only have to butcher the isolation and then go on with the new library. In it's simplest form the isolation is just a redefinition of the function names and in the other extreme there is also some data conversions in between.