Maybe it's not popular with purists. I know the retiming trick people try to do doesn't work well but I think Mike's modified bobbin case is different.
To me it always depends on the modifications and why they were done.
If a person is planning to actually sew with one of these machines it makes sense. This mod allows the user to use all types of needles instead of only the two available new without having to hunt down NOS needles on Ebay.
I'm guessing the mod is an enlarged slot in the bobbin case to clear the 15x1 needles. I don't think that it would prevent the user from using the stock 206x13 needles if they chose to and wouldn't affect the cosmetics of the machine. Seems like something Singer should have done from the beginning since the 15x1 needle was pretty well established by the time these machines were made.
Rodney