You're not missing out on much, to be fair. MpM fares never seem to be a huge saving - if any - over the lowest available bucket class (and they only seem to be available when those cheap fares are too anyway), but they are very restricted - so it's difficult, if not impossible, to do an airport upgrade and they cost you miles rather than earn you some for the small delta in cost saving you make.
As an example, I randomly looked up a flight to New York, flying out on the 12th Jan, returning on the 19th Jan (so you include the all-important Saturday night). MilesPlusMoney fare is £472 plus 5000 miles per adult. The N class fare on the same flights is £472, but it earns you 3458 base miles (more if you're Silver or Gold, obviously).
It does kind of make the whole MpM fares a bit of a joke.