ok, hard rerolling 20+g trying to get it and sacrificing income probably isnt a good payoff. however, if the cards DO happen to eventually give them (with whatever your econ/reroll strat is), i fully reject the notion that the cost of the cards does not equal the benefit of having them on the field. the health and damage increase is great to have. will it absolutely carry you? probably not, but it might believably be the difference between living from a spell or two or something like that. if they're a core unit for your comp, theres no reason to NOT buy them when they show up. that level 3 kassadin/darius/garen, even shit ass MORD, can tank a little longer and thats their JOB.
the only counter argument i can see, is about saving bench space while you try and influence the roll chances for higher things or denying enemies. but even then, thats not that significant, and you can always sell them to make bench space for a stronger unit that you're trying to level and still get full value.
again, compromising your income to try and force a 3star early is generally a bad strat, but i dont see any real reason to not buy the ones you need whenever they show up.