Re: (Nutaku) Milenium War Aigis (English Version)
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.
That's... not why Event units don't work for Premium rolls.
The main reason they don't work is because, due to the fact that they're Event units, most of them are already weaker and have a higher cost than Premium roll units. Odette would be one of the extreme examples, she's a Platinum Mage and those usually come at a 31(36 after CC) cost, but Odette starts at 35(40) and in addition to that, her skill goes up to
16 (5% to 20% Magic Resistance). If you end up rolling her from Premium, you pretty much got a dud, she's nearly unusable and her skill's virtually useless.
Giving people more Crystals is just a terrible idea, financially, considering they already hand out a ton, it's just going to make them lose potential buyers.