FWIW, one reason we've wanted to override the allocators is that the one that comes with nim 1.2 and the "traditional" gc is quite inefficient when it comes to memory consumption / fragmentation (we've been able to halve the resident set size simply by replacing it with `malloc`) - haven't checked ORC though.
The other reason we've wanted it is to get access to C tooling: valgrind, leak detectors etc - even with a GC, they're quite possible to use.
