I Know My First Name Is Stephen
Karen would not sing the work lead while laying down a drum track for two reasons:
1. The work lead would 'bleed' into the drum mikes, making it impossible to mix the drums later during mixdown or add effects without this affecting the lead. Richard would probably never have had her doing both simultaneously as the work lead ended up on occasion being used as the final lead vocal and he wouldn't have wanted to take the risk.
I forgot to add "and vice versa". The drums would leak into Karen's mic making it impossible to isolate her vocal during mixing.