In the classic rock era, so it seems, it was a pretty frequent (if not universal) occurrence that the lead singer of a band would step aside for one or two songs an album and let someone else sing a song. All four Beatles sang lead on various songs, as did all three members of Cream. In the Who, Pete Townshend sang a few songs on every album, and John Entwistle usually sang the songs he wrote himself. Fleetwood Mac, at its peak, had Stevie Nicks, Lindsay Buckingham, and Christie McVie all handling singing duties. In Blue Oyster Cult, guitarist Buck Dharma almost sang more songs than the “official” lead singer Eric Bloom, and even drummer and bassist Alan and Joe Bouchard sang a lot of the time.
In contrast, I can’t think of a single rock band of the last 25 years or so which has anything other than one person who sings lead on all the songs, not stepping aside even for B-sides or the deep album cuts. The closest things I can think of are duets, songs where someone who’s not a member of the band is explicitly billed as a guest artist, or pop or rap groups (like the Backstreet Boys or the Black Eyed Peas) where none of the band members play instruments anyway.
Why the changing trends? Or am I just not listening to the right material? Are there any rock bands that do do this kind of switching off?