For example, where the standard version of a program might be limited to handling lines less than 4096 characters long, the GNU version is likely to handle lines of any length that memory can hold. Unfortunately, when you use these extensions, your program becomes less portable to other platforms. To make a program generally useful, the GNU C library should be ported to any platform where your program might be useful. On the other hand, writing good software that requires the GNU C library may encourage the further spread of the GNU C library. You can spend less effort getting around library limitations.
Buggy libraries can waste a lot of a programmer's time, as veteran programmers know.
Since the GNU C library has a reputation as a good implementation of the Standard C library, with useful extensions, you may be doing all your fellow programmers a favor by encouraging the spread of the GNU C library. Another reason to encourage the spread of the GNU C Library is the very fact that it is free software. It can be a tremendous help to be able to read library source when you don't understand what a library function call is doing. The linux C library is based almost completely on the GNU C library and will probably be merged with the GNU C library eventually. This does not imply that writing programs under linux requires or encourages writing non-portable programs.