For example, I've done some work with audio or video. Nobody working on that goes straight to malloc on every packet or frame. It'd just be asking for pain.

But a general purpose allocator doing its own free-list on the assumption that libc is going to suck? I think that's outdated. If you do want to support it, I think it's better to allow the caller to replace the allocator through a function pointer, rather than just do it by default in a library.