Miles don't expire, but an account with no activity over a period of time may be closed (thus forfeiting accumulated miles).

So it's confusing to say that miles don't expire, when you can lose all of your miles if you don't earn or burn.

The importance of the distinction is that when miles expire you need to use them within a period of time or those miles will be lost (but not other, more recently acquired miles).

I actually like the change, but it can be confusing to some members.

Note that 'expiring miles' (or actually credits) is what Southwest, JetBlue, and AirTran have ... use credits in 12 months or lose them. Note that you don't lose all your credits necessarily ... just those that are more than 12 months old.

Starwood has an interesting policy -- points don't expire (note that there's a wrinkle that deals with Starwood Vacation Ownership incentive points) but an account will be considered inactive and can be closed if there's no earning at a Starwood hotel or with a Starwood co-branded credit card within a 12 month period.