It would be helpful if stewards can change what usergroups can do on wikis just like how they can modify global usergroups. This way stewards can for instance enable/disable file uploads or change which usergroups can upload as well as other fine-tuning as deemed necessary by consensus.
Global user groups are managed in the database, hence a front-end feature through CentralAuth can manage them.
However local user groups are defined in the PHP server configuration. There is no such feature in MediaWiki so this can't be done until such system is created in the MediaWiki software itself.
Useful feature to have indeed, but it's within the frameset of having an interface to a bunch of configuration changes as stewards. It would be nice to have, but not a pressing need, I would say. Marked as low, enhancement.
Correcting Product to MediaWiki, since feature doesn't exist.
I will point out that if there is eventually agreement to disable local uploads by default on Wikimedia projects (and possibly restrict to a user group like sysop), developers may receive hundreds of requests for related configuration changes. It's possible the priority for the bug will then suddenly be a bit higher... Maybe a bit of planning ahead here of how that situation would be best handled (not necessarily by this bug, I'm no expert) would not go amiss.
@Rd232: Moving it back to Wikimedia/Shell requests. This request is to grant Stewards the user right, adding a feature to MediaWiki is a completely different subject.
I've filed bug 36857 for the implementation of the feature in MediaWiki and added it as a dependency for this one.