Session singers are often there to mimic the sound of the lead singer and boost them in the choruses. That's what often makes certain ones particularly employable as opposed to only providing backing vocals.Re: ghost singers in studios to cover for a star's weak vocals, poor range or incapacity through drugs and booze - it definitely happens. It must be easier than editing a bad vocal and it probably just sounds better. The ghost singer gets a credit as an additional vocalist, their session fee and whatever else they can negotiate. They make their money by being quite anonymous a lot of the time and using this to be able to work on lots of different projects.
I read about Mandy Smith's album she did for SAW. Most of the singing you hear came from Suzanne Rhatigan. Mandy was right down in the mix. When Take That were recording their early albums, Jason's vocals were buried with someone else's tracks because his voice wasn't good.
Mandy Smith's vocal is certainly discernible on the PWL tracks she did, but also doubled all the way through by someone else, be it Suzanne Rhatigan or Mae McKenna. Mae particularly cleaned up doing loads of sessions there, back in the days when there was loads of recording work.