There seems to be at least one reason to have a non-GC option: You want to write code that integrates with an environment that already has its own GC and you want that to decide things. For example you want to integrate existing libraries in C/C++/D/Go/Rust/whatever with a Lisp process, since there's lots of those libraries and they do useful things, and you don't have a Lisp library for that (yet). In that case having one point of deciding when to deallocate things seems to be better than having two of them.
