If redefining the value of 1 BTC could help adoption even the tinniest bit, it is worth considering. Heck, mega-concerns, like Coca-Cola, are marketing their products in far more subtle ways than that.
When you buy a can of coke, what does it say on the side? Around my parts, it says "330ml". I buy drill bits measured in millimetres, my national currency already has subunits, and so on. I guess I don't see the issue with mBTC - people will get used to it if they need to.
Far from "helping adoption", the confusion introduced by changing the unit of denomination would almost certain hinder adoption. Moreover, the headaches would likely be felt most strongly by newcomers and those who are bad with numbers - precisely the crowd that find Bitcoin intimidating already.
It is a bad idea.