I like guest strips. It's understandable that when you do this every day for a living, you sometimes need a break, especially now that you've got a new little one to divide your attention.
Guest strips not only allow the artist to take a break, but it gives the readers a chance to see a familiar world from a different angle, not to mention giving us a sampling of different artists' comic styles to find and enjoy. Several of the webcomics in my reading list were added due to followed links after a guest strip.
So it's really a win-win-win situation. The artist gets a break, the reader gets new reading material, and the guest artist gets new site traffic. It shouldn't be too often, but I don't think a guest strip week, say, twice a year should be anything to get upset over.