I'm not by any means an expert on the ins-and-outs of image formats, but I believe XCF is more or less GIMP-specific (with a few exceptions, like Paint.Net and a smattering of GIMP forks). Which means you'd be pretty much forced to use GIMP in order to create/edit those files. The PNG format, on the other hand, is a much more universal format. PNG files can be created/edited by the vast majority of even the most basic image editors.
Speaking as an artist who uses Photoshop primarily, I think this would not be a good idea, because it would scare away potential contributors by forcing them to work with an image format that they may never have heard of before, let alone used before. And even if they weren't scared away by that, now they'd be forced to either convert every XCF image to another format in order to work with it in their image editor of choice (and that conversion process, besides being arduous and annoying, could also introduce other issues, like missing transparency information, compression, artefacts, etc. ). Or they could just give up their favorite image editor and use GIMP, which most non-Gimp users probably won't be willing to do.
There's also the matter of file size (is a layered XCF larger than multiple .png's?). As I said, I've not used XCF much, so I can't comment on that. But it's another important point to consider.
Honestly, I very much doubt that XCF support will be added to Wesnoth any time in the foreseeable future. But if it ever is, I would think that it would be in addition to the PNG format, not in place of it.
"I'm never wrong. One time I thought I was wrong, but I was mistaken."