It's not that it's a proxy for memory use, but FD is its own resource, I've seen many software with FD leak (i.e. they open a file and forget about it, if they need the file again, they open another FD) so this limit can be a method to tell that it's leaking. Whether that's a good idea/necessary depends on the application.
Then account for _kernel memory used_ by file descriptors and account it like any other ulimit. Don't impose a cap on file descriptors in particular. These caps distort program design throughout the ecosystem
