Back in my shop days we were so busy there was no other option. Bikes we sold first, everything else after that. During peak season we’d run about 2 weeks on our own bikes. Inconvenience repairs like flats and chains we’d do while the customer waited regardless of where the bike came from.
On Saturdays we did no repairs. We did no full rebuilds in the summer, there was a 2 month wait for those in the winter. (Upstate NY)