I used to prefer C89 but learned the hard way that there are quite a few "bugs" in the standard, where it is ambiguous or otherwise fails to give a clear answer. So C99 is my go-to standard these days, even though I only care for a subset of its features.

"nonarray object" definitely sounds like such a bug. I think the intent of this clause is clear: it is meant to make sure you can always pass a pointer to a single object that treats its argument as a pointer to an array element, and does pointer arithmetic on it. One of the most common construct is simply looping over an array by incrementing the pointer, and this must work without producing an overflow when the pointer points past the array, the way it's conventionally written. If it weren't for this clause, passing a pointer to a single object to be treated as an array of size 1 would break a lot of code. Going further, is the object allocated by malloc an array or a nonarray? That would then be a critical question to ascertaining the correctness of most code out there.

And I cannot think of any reason why only pointers to nonarray objects should be usable in this manner.