I suppose it saves space if you're sending a message on a mobile phone, where the number of letters you can include may be limited, and maybe it's quicker when you're keying in letters on a tiny numeric pad.

But in a context like the Mudcat it is much much slower to read, which gets in the way of communication. That goes for most of the three letter acronyms as well. Instead of being able to read fluently you find yourself having to decode the stuff.

Also, I think it looks ugly.