It seems rare to have the lead singer record/overdub their own backup vocals in professional settings, but does the quality actually suffer, and does anyone know of any side-by-side comparison?
Honestly it’s rare to have a band with a singer that is capable of recording all the backup vocals. But plenty have.
A lot of the time the backup vocalists are the other band members and they want to record it the way they perform it. Also people have different tones in their voice and it adds color to the song. Same reason guitarists will record rhythm with one rig and leads/solos with another.
Here is a piece from Wikipedia
In the recording studio, some lead singers record their own backing vocals by overdubbing with a multitrack recording system. A multitrack recording system enables the record producer to add many layers of recordings over top of each other. Using a multitrack system, a lead vocalist can record his or her own backing vocals, and then record the lead vocal part over top. Some lead vocalists prefer this approach because the sound of their own harmonies will blend well with their main vocal. One famous example is Freddie Mercury of Queen singing the first part of "Bohemian Rhapsody" himself by overdubbing.1 Patrick Stump of Fall Out Boy, Tom DeLonge of Angels and Airwaves, Wednesday 13 in his own band and Murderdolls, Ian Gillan of Deep Purple, Simon Le Bon of Duran Duran and Brad Delp of Boston also recorded lead and backing vocals for their albums. With the exception of a few songs on each album, Dan Fogelberg, Eddie Rabbitt, David Bowie and Richard Marx sing all of the background vocals for their songs. Robert Smith of the Cure not only sings his own backing vocals in the studio, but also doesn't perform with backing vocalists when playing live.
This is a very late answer, but I thought I'd also like to contribute to this.
You will be saving in terms of hiring an additional person, but you will still be billed for studio time.
I know Sam Smith did his own choral layering for 'Stay with me'. This brings a unique sound where the overtones stack on top of each other giving it's own unique sound.
Each person has a unique tone/timbre to their voice. These in turn produce their own overtones. When two voices sound together they can either have clashing or blending depending on these overtones. It also shapes the sound into something new again.
So depending on the type of sound you want this can be a very difficult process in finding the right voice to serve as a backing vocal for someone else. Will they blend or clash? Choral or ensemble singers will be a good start if you are looking for someone.
I had forgotten this question existed. I discovered the answer to my own question by experimentation.
For the song in question, we recorded both the lead singer and the other vocalists to sing backups. We then A/B'd both versions, one with the singer herself as the backup, and the other as using the other vocalists for the backup.
The diversity of vocal quality of the latter made it a clear winner in my opinion, so we went with that. I also made it a point to make the backup vocalist with the more "characteristic" voice sing the higher part of the harmony to stand out more, and I'm glad I made that decision.(chorus starts at 1:01)
A lead singer should have a distinctive voice that sticks out. A backup singer should blend in. You'd not want to use autotune on backup tracks.
It's not like there is no use for reusing even a characteristic voice, but then the effect tends to lean more into that of a chorus one rather than lead with backup. A well-known example is Queen's "Bohemian Rhapsody", a studio production on the album "A Night at the Opera" multi-tracking Freddy Mercury as singer.