It's not that sticking them in the premium rolls can't work, it's that it can't work with the current distribution of crystals. Keeping event units to premium rolls only could work if they increased the number of crystals players received according to their performance. Like adding that the first 3-star completed H level daily every day rewards a crystal. Or, to give starting players an equal chance, give the first 3 star of each tier a progressively higher chance to drop one (E - 25%, N - 33%, H - 50% or what have you. Too lazy to do the math to optimize it). Also, with the increased frequency of people rolling, the temptation to drop a few dollars for a couple extra rolls gets more and more attractive. We're pretty much playing slots, and anyone who's played will tell you that it's not an attractive game until you try a couple times and win just a little bit. Increasing a normal crystal distribution also keeps players on a bad luck streak from being frustrated to the point of quitting. The guy who spent $10 on two silvers is less likely to be frustrated if he knows that he'll get another shot for free in 5 days rather than 25.
