Two reasons. 1. I thought the consensus in this thread was than ISBNs don’t fit in catalog numbers and putting them there is just a hack for audiobooks until MBz supports a separate field for them. 2. I don’t think they are catalog numbers in the sense MBz uses that term. Catalog numbers are supposed to be for looking up within the catalog of the associated label (or for [no label], the artist), right? ISBNs are for global lookup like UPCs/EANs, not lookup within a catalog. Libraries using ISBNs seems more comparable to stores using UPCs/EANs to me.
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A catalogue number is something used to identify a release that you hold in your hand. If the ISBN number is in the catalogue number field it can then be searched for. Some labels do use these are their own catalogue number. And you can list multiple catalogue numbers.
Until the ticket goes through I don’t see any reason not to do this. Especially as once the ticket is done it makes it much quicker to find them to move across to the new ISBN field (if it appears)
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I recently entered an ISBN as the catalogue number. It was the only number on the release. Many audiobooks only have an ISBN, at least in Germany and Austria.
I don’t see the harm in that, and, as has already been noted, the ISBN serves the same purpose. But I haven’t entered many audiobooks yet (one), and if the decision is against it, I can easily fix that. 
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