It was necessary. You need vets in the AHL, and they were too good for AHL deals. Greening is the one guy you could argue was only worth an AHL deal, but even he was very good in the AHL last year and likely would have got an NHL deal elsewhere. Marlies need his skill (especially with Gauthier down for a while) and leadership.
Regardless of whether we had spots, Dzierkals was only worth an AHL contract. He has been underwhelming both years in the QMJHL and potential is the only reason he is even getting what he got. Even if one year ELC's were a thing, he probably would not have got one. 3 years... Definitely not. He's far more likely to bust than to turn into anything at this point, and right now his chance to even play a depth role on the Marlies are slim to none.
Luckily he still has a ton of upside, but try telling that to Korostelev. It's not like he is much different... Both are going to need big years to warrant an ELC, and both are going to have to show it at lower levels than the AHL; ECHL for Dzierkals (if he decides not to go back to Europe) and OHL for Korostelev.