base tried to do one too many things – automatically handling loading modules but also allowing establishing inheritance from classes already loaded (possibly from a file whose name wasn’t based on the module name). To sort of make it work, there was some hackery that caused surprising results in some cases. Rather than break backwards compatibility, a new, replacement pragma parent was introduced with cleaner semantics.
parent will be a core module as of 5.10.1.
Update: forgot that base handles fields (if you are using the fields pragma), which parent doesn’t do.