Well, I guess it depends. With an M of 6-10 you still have some room to make plays. Also, your M with respect to the rest of the table is also important. In the later rounds when the blinds and antes get to be large, just about everyone's M will be low and many will feel the need to make a move.

What I believe Harrington was saying was that with higher M's you have a wider range of hands that you can speculate upon and more options in regards to betting and deception.

There are several people in the MTT tournaments forum that could answer this better than I can. Also if you search that forum you'll find several discussions regarding the pro's and con's of M.