Every pre-paid card I've seen has in the fine print an expiration date. If there are some that are totally open-ended, that does change the argument a bit.
Those expiration dates are quickly becoming illegal. I don't know how it applies to pre-paid minutes, but for gift cards, most states have eliminated expiration dates.
I don't know about other systems, I use a Tracfone, and while I have to buy a new card every 3 months, the minutes roll over - I'm up to over 7 hours now.