Wow! I'm really surprised by the lack of travel hacking in this post. I thought all MMMer's were hardcore travel hackers.
A few years ago I would have said the same thing.... well, almost. I don't like the term travel hacking.
I've played this game for a number of years. Enough pyjamas to last for a life time from international First class flights. Spent less in flights then others spent on parking their car at the airport. Still, it is only a period of time in life for several reasons.
Most traveling on miles have to decide whether they want as much travel as possible or as much perceived value per flight. Since European airlines charge YQ on award tickets, but US airlines do not, you will find that many European members value luxury over number of flights. Luxury is something you can get used to and easily rationalize why you should pay for a higher class even when you don't get an upgrade with miles.
The sweet point of traveling is two persons. There are offers that give you a second ticket for free or little extra. Most airlines do also release two seats or more in most cabins. Add kids to the mix and it gets a lot less compelling. Seats are harder to get by. Travelling is limited to school holidays and high season. Luxury travel is not relaxing when you spend the time calming down a small kid.
It is risky. There is no guarantee flights are available, airlines stay around, prices don't change, offers are honored, miles are credited... If you can't afford to lose your miles, you shouldn't collect them. If you depend on specific redemptions, you need to realize there is no guarantee.
This game narrows the focus. Maybe taking the train or finding a vacation spot close to home would be financially more effecient. When you have miles, every vacation looks like an international flight.