Well, that sounds more like worrying about a single point of failure (e.g. that power source that can take out several drives at once) than what the redundancy is designed to protect against, i.e. uncorrelated failures of one of the drives in the array.

Of course having no redundancy at all during a rebuild that takes more than a negligible amount of time is still worrisome, but that's why we have multiple-redundancy raid schemes.

But you still have to untangle your concerns. A single point (or cause) of failure has to be handled differently than increasing probabilities of encountering independent UREs on separate drives during rebuild.