I would argue that mules should indeed be catalogued (somewhere, at least, so they are documented, whether or not in SCWC), simply because (a) they turn up and people need to know what they are and (b) they are the result of human error rather than the sort of random accident that leads to die cracks and the like.
On a New Zealand forum (nzcca.com/forum) I recently did some "thinking out loud" and came up with the following breakdown of "non-standard" coins. Examples refer to coins in the NZ series, for obvious reasons:
1. Varieties: changes brought about by the use of different dies or switching of dies in mid-run (e.g. the 1984-85 "recut die" types, 1988 type I and II 50c, 1950 type I and II halfcrown, the "mint set" vs. circulation strike differences that we have often discussed);
2. Errors: the accidental use of an incorrect die (e.g. "strapless" 1956 1d and 3d, 1957 6d; mules, other non-standard variations caused by human error and often involving an entire die run or more);
3. Faults: cuds caused by filled or cracked dies, other die cracks, off-centre strikes, straight "accidents" incurred as part of the process of using proper dies, with no human error involved other than possibly leaving a die in place for too long or not clearing it of excess oil!
The first two can generally be catalogued but the third - and this is only my view - tends to cover such a multitude of possible problems and locations that there can be no end to them, depending how closely you look. I would suggest that only those that are both quite spectacular and happen to turn up with any frequency cross the boundary into becoming cataloguable and are likely to be sought by those who don't have a particular focus on "errors" in general. Examples would be the 1967 No sea 5c, 1965 broken wing 6d, 1942 and 1958 broken back shillings, 1955 2-dot 3d, 1942 1-dot 3d, 1999 wart-nose 5c, 2008 butterfly lips $1.