I have found from time to time, when using Data.Sequence, that a list-like function that I expect to be there is in fact not present and its absence is not mentioned; it always turns out to be implicit in one of Sequence's instances. Examples: map, fold, (++). This is in principle no different, except that MonadZip is apparently relatively obscure and, possibly, not the "right" class according to Tony Morris' link.
I can't tell if this is an argument for or against the proposal, but it does seem to reflect a choice to populate the Sequence API via instantiating standard classes rather than writing standalone functions (with possibly conflicting names, not that this is a new thing with these container libraries). Both map and fold have Seq-specific indexed variants that are
not part of those classes, and if neatness is what the module authors were going for, then this kind of variation is the only thing that should actually appear in Data.Sequence itself. It does impact the usefulness of the documentation, though. I think that's the real issue here.