It may well -- would give us the ability to push it onto other usergroups if we need. The primary reason for using the global was that it's much more clear *how* to disable email-blocking, as this is likely something that many wikis will want to disable (at least, it's more clear than $wgGroupPermissions['sysop']['blockemail'] = false;). Everyone I asked about it felt as indifferent about it as I did though -- how 'bout we make a hybrid of the two? Have the global that will disable it regardless of who has what rights and the right itself which is also needed to use it?
If you can't decide how to do something, let the user choose. It's always seemed a good philosophy to me.