If I understand correctly: [[mw:Manual:$wgDisableHardRedirects]] is either all or nothing, $wgRedirectSources is only about incoming redirects, $wgInvalidRedirectTargets is only about special pages.
Enabling hard redirects for everything (i.e. for all local interwikis, cf. bug 40352) is or can be too much. For instance, all Wikimedia projects are "local" to each other, but it's not very appropriate to automatically redirect a visitor, say, to a Wikipedia in another language.
It would be better to allow whitelisting for specific local interwikis, which could be used to enable hard redirects to specific "particularly close wikis"; and maybe also for specific special pages, if one wants to use a specific special page (maybe added by an extension) without worrying of all the additional possible special pages (as with $wgInvalidRedirectTargets).