It's not only meaningless due to T-Mobile being untrustworthy, it's meaningless if it's for a specific network speed or volume. For example, would 3G and a 2GB plan for $50 lifetime guarantee be useful?
It's partially sleight-of-hand, but it has some value. It effectively means that you've got a couple years of leeway to find an alternate plan without price changes in the meantime if the speed/data become unworkable or a network shutdown becomes imminent.
