The particular screws he is trying to remove have been locked in with red loctite in the crankcase for the past 17 years and are notoriously difficult to remove, i have JIS screw drivers and regardless of how good the fit is you cant get them to budge by far the easiest straight forward way is to use an impact driver. An impact driver with a JIS bit would be best but not necessary, the Philips bit gives enough to break it free.No, using non JIS bits on JIS screws is getting by. It's likely using American wrenches on metric fittings. On some sizes, you get lucky and the fit fairly well. Or, do you just say screw iit and grab the adjustable wrench for everything?
If it cost him 200.00 for the correct bits, then sure, it might be time to get by, but when the correct bits cost very little, or less, why not use them?
It's also nice when you have proper fitting bits and don't have to start smacking cast aluminum cases with a hammer.
The impact screwdrivers have their place. That place is usually after someone has damaged the screw with a Phillips bit.