That would depend on the application. The convenience isn't just a smaller binary, but also being independent of the distros glibc version (being on an LTS distro and not updating often, I have experienced incompatible glibc versions often enough). The megabytes shaved off are significant if the final binary size is < 1MB, which is a completely different size class.

And not all applications are performance sensitive. Something like UNIX's bc command would benefit more from having easier compatibility and a faster startup than more optimized allocators and string functions.

In the end, it ~doesnt even matter~ is all trade-offs. The nice thing is that it is up to the developer to make the decision of which libc to use, so everyone gets what they want.