My bike goes to Triumph, Woburn for anything mechanical/servicing/MOT, whatever. Never been charged for the privilege of riding another bike and I would flat out refuse to pay (unless I binned it obviously). They've got their fair share out of me and it's time to return the favour!
I've had to pay a tenner at a non-triumph dealer a year ago when i dropped my bike in to be serviced. To be honest, i don't mind paying a tenner for a courtesy bike - after all the dealer doesn't have to have a bike for you to taz around on for the day. The upkeep of that bike isn't free, so a nominal tenner towards tyres/chain and sprockets, i don't really mind.
I didn't get that far - just a little shocked as never been asked to pay for a courtesy bike before. They charge £90 per hour or something for labour and add some crazy profit margin on their oil - that should more than cover a tenner for a courtesy bike. I have no problem with topping up the fuel and paying the excess if I drop it.