Ever since The Beatles appeared, Rock bands have been expected to be not just musicians and singers, but songwriters as well. The thing is, writing a song is a very different talent than performing one. Being a good musician and/or singer does not necessarily and automatically make one a good songwriter. There are FAR more good musicians and singers than songwriters, imo. I would much prefer to hear a great song performed by average musicians and singers than an average song performed by great musicians and singers. But then I am, as Dylan once said, a "song & dance man". Hold the dance ;-) .
For me, the song and its writer are akin to the movie script and its writer, the musicians and singers the actors. Sure, that's an over-simplification, but you get my point. One songwriter performing anothers song is the highest compliment that can be bestowed upon one. That's why when Merle Haggard recorded Iris Dements "No Time To Cry" I knew she must be something very special. Merle was SO right! Kasey Chambers, another gifted songwriter and singer, has also recorded the song.
There is another pair of songwriters currently touring together, Jimmie Dale Gilmore (The Flatlanders, solo) and Dave Alvin (The Blasters, solo). Not on the same level as John Hiatt and Lyle Lovett, but darn good, both of 'em.