The problem with using an apostrophe unnecessarily for plurals of dates ("the 1940's), acronyms ("SNAFU's), etc., is that it quickly spills over into ordinary words. E.g., the humongous, expensive 3-D sign in a Sears store that read "SHOE'S FOR THE FAMILY" or the grocery store advertising "Avocado's" or a menu offering "Hamburger's."   

And, as I said, if you start thinking an apostrophe means plural and for get its real purposes - to indicate an omitted character or characters or to indicate possession - it's very easy to write "during the 50's," which not only has an unncessary apostrophe but omits the one that should take the place of the "19."