It isn't bull. And i wouldn't offer any advice on your experience of just one example.
You have been lucky or didn't have a slipping clutch to begin with. The metal pressure plate and cover plate get over heated and by simply fitting a new friction plate will mean doing the job again in 4-6k miles when it slips again. And if it does chances are you'll have to replace the "new" friction plate.
I'll only replace the complete clutch unit when doing a clutch change.
I will fit just a friction plate if the customer wants but offer no guarantee if it slips again within a few thousand miles.
If i fit a complete clutch unit then they're covered and it's all down to me