On Smith's site the OAL is listed as 6.1, where did you get 5.42?
Product: Smith & Wesson M&P SHIELD
I think the M&Pc's will sell to people who want the ability to use full size mags, like myself. But I know on various forums there has been call for a single stack M&P for a few years.
I don't see myself buying one only because I already have the 9c and can conceal it fine.
Smith seemed to have no problem selling the 3913 and 6906 at the same time. I guess time will tell.