Why you might want to take the first exit
Risk the Newbury bypass
You start from scratch
In the beginning, the instrument will be totally impossible to play. It probably takes between three and six months before it starts to make sense, and there is a good chance that at some point you wont be able to play either the 6-string in conventional tuning or the M3. If you want to be able to make the transition successfully, you have to be very determined and very patient.
You throw away part of the tradition of the guitar
A lot of the things you hear guitarists play on 6-strings can be very hard to execute on the M3. The basic open-string chords, such as D, A, and G, are gone. If you are hooked on the very specific sounds you are familiar with from records, then there is no point in changing the tuning of your instrument, it will just make your life more difficult. In addition, you cut yourself off from some great tutorials. Books that include fingerings and tablature are useless. So if your preference is for the traditional rather than the adventurous, the M3 is not for you.
You have to make an effort to find equipment
The selection of 7-string guitars is severely limited, and you are unlikely to come across more than a few of them in even large music stores. To the best of my knowledge, no factory-made 7-string acoustics exist, and no midi-gear either. Strings for this particular tuning are not readily available. When you buy them as singles, they are more expensive (even though you can save some money by ordering them over the internet). Basically, it takes the fun out of shopping.