A CoA is fairly standard in the art world, especially for limited edition prints. You could ask they same question about why they bother? The ones that I've seen don't mention the print's archival properties. I think they're mostly done to document the precise conditions of the limited edition, since there are often a number of artist's proofs as well in circulation. Perhaps also the date that the print was made, since not all prints in an edition are necessarily produced at the same time.
I've occasionally done limited editions, and given the expectations created by print makers, I generally offer a CoA in such circumstances. In the case of photo prints, it can be helpful to document the extent of the limitation, as photographers generally reserve the rights to reproduction of the image in other forms, sizes etc. I'd also include the sort of information that Alan indicated. Why bother? Partly expectations and partly to cover myself when other reproductions are around.
I don't like limited editions either, but I sense that there's a lot of pressure in the photo-art market to produce them. I overheard a conversation in a gallery recently, in which a prominent member of the local photo-art community said that she had always wanted to buy a print by the photographer whose exhibition they were viewing, and now that he/she had produced a limited edition, she would. I was a little stunned, and had to restrain myself from asking why? If you like a photo print and want to support the artist, why not just buy it? Do you really think that the photographer is going to sell out the edition and by limiting it you're getting something more valuable? That's most unlikely, unless the size of the edition of very small or the photographer very famous, or a great self-marketer (e.g. Peter Lik). It's most unlikely that you'd get anything like you're money back were you to try and sell your print in the secondary market, except in the case of very famous prints / photographers. End of rant.
