I have one average at best grinder - it grinds fine enough for a decent espresso.... If you were in a commercial environment I would say yes, you probably would need two (or more) as time is important to you. However, why do we have 2 (or more) of nearly everything.....because we can. We have about the highest standard of living in the world and the material wealth to go with it.
I suppose that as people get upgraditis and find themselves with 2 (or more) grinders then it would be handy to have the back up. If you are going to put Decaf and regular through or have a couple of SOs on the go at a time then it might be annoying to clean out your grinder so much. But to actually need a second grinder...I doubt it. Just need to clean it more often if you are particular or caffeine sensitive (which many probably are).
Sorry, probably went off on a bit of a tangent about needs and wants. A friend of mine has two (Mazzer and Sunbeam) as a result of upgraditis - not decaf consumption, he only uses the Mazzer and has the sunbeam as a back up if the other is out of action (which hasnt happened yet).
Others will be better able to answer your question.