Right now, ASSS will search for modules in localbin, bin, and corebin (in that order), and this seems to work reasonably well.
Unfortunately, having the asss binary itself in either localbin or corebin presents problems to the launching scripts and the automatic backtracing.
Modifying the launching script with the correct path of the binary isn't difficult, but the backtraces still present a problem to people wishing to run asss from other locations.
I believe the rationale is that a zone host could provide each zone with read-only symlinked directories for corebin. This would allow the zone host to upgrade all the zones on the server at once.
Grel used this in combination with chroot with Hyperspace on SSCX-A (back when SSCX existed), but I think it may have been before the bulk of the core modules were pushed into the asss.exe. The scripts he used to do this are probably still kicking around in the source tree.
At this point, it's probably irrelevant. No one uses the functionality because it's so obscure and this bug prevents serious usage. Perhaps it's best to just quietly remove the feature entirely.
I see no documentation for these dirs. From the behavior and names, bin is the default location for core binaries, and I assume localbin is for user modules. Why have corebin at all? Why would the asss binary end up somewhere else?