In most cases, undo just provides a longish edit summary and in reality, it doesn't have an advantage in comparison to normal editing. I'm against such a change, since everybody can and should edit. This is just a minor shortcut that can't provide much damage.

This is just an optional deterrent, having undo can and will make it easier for vandals. I see this several times a day, more times than I and you have fingers and toes combined. This is optional, and undo would still be there by default. More places than Wikimedia use this software, and sometimes variations to prevent damage are needed. As for your comparison to "being a shortcut" and "everyone can and should edit", I could say the same thing about "rollback" users, why have the separate right if you can just as easily go back into the history and click down a few revisions and resave the page? Yet the group exists because it's a convenience requested by those who deem it necessary.

Well, same could extend to rollback, except that rollback can revert quite a number of revisions in some cases. But, yes, that doesn't change the fact that everybody can mimic "rollback" (same goes for "undo") and even go to the lengths of faking a "rollback" edit summary. The point is, "rollback" is given out as a shortcut. However, you want the shortcut of "undo" taken away. The difference is that "rollback" was available (in its true shape) only to admins, whereas "undo" is available to anyone. Sanctioning rollback power to certain users isn't in the same category, IMO, as forbidding "undo" to non-admins or whomever.

Then again, as it seems you're proposing this for non-Wikimedia wikis, i.e. generally for MediaWiki, it just might happen, but I don't see merit nor valid rationale behind it.

No special abilities in undo.