Some interesting points there. I think that there will always be people who want jobs doing at the last second but if it is an MOT or service, ie something that they could have planned, then they are often the kind of customers that you might not want anyway - the type where everything is last minute and no money!!
If you can build up a reputation for good work as Tony suggests (in my view, price is often less important) then customers will come to you even if it means booking well in advance.
Lots of dealers have been struggling over recent years but as far as I can see it's the workshops that keep them afloat - high hourly rates for mostly easy work compared to expensive showrooms selling new cars with very little profit margin.