I thought the purpose of pre-paid credits would be obvious: allowing parents to load up their kids' accounts with specific amounts of money so that they don't have to use their parents' credit cards and so the parents know for sure that they won't go over their allotted amount. Yes, this includes teenagers as well as younger kids.
I think the chances of Steam doing away with direct payments are just about nil. Never gonna happen.
But if they're going to implement pre-paid amounts along with direct payments, why not go all the way? What I would have done is announce discounts for purchasers who buy more than a specific amount of pre-paid credits. The discounts would have to be kept below the amount that Valve is making extra from leftover credits along with the interest being made on money exchanged for the credits that sit idle for significant periods of time, but turning these sorts of discounts (even if they're relatively minor) into a marketing tool is right up Valve's alley, and it would take the sting out of the inevitable "wasted" money represented by leftover credits.