The pros are that it keeps the tweed clean otherwise it can pick up a lot of dirty marks. Can't think of any cons. I recently varnished a tweed amp with a brushed-on single coat of water-based poly varnish and that was easy enough to do (and I could do it inside).

You can of course use a tinted lacquer to make it more yellow, like Fender do with their lacquered tweed cabs, but clear will provide protection if you want it to look more subdued.