I agree with you that Sorcery tree seems somewhat useless or weak compared to other rune trees, especially Domination. However, Nimbus cloak seems to be cool because it gives you something more "readable" than Ultimate hat. There are many mages (and let's not forget supports who use Sorcery as either their main or their secondary rune tree) who can make use of the new rune, just to mention a few:

I would also like to highlight the fact that Nimbus cloak is not a keystone but a simple rune, which means that many other champions, who would usually pick something else as their secondary tree, can now pick Sorcery and Nimbus cloak as a niche rune pick. I'd like to mention a few champs who sometimes can make use of it:

You are, however, totally right in the fact that there are many mages (and also other kind of champions too) who don't profit from Nimbus cloak at all. These champions are the ones whose ultimate either roots them to one place or makes point-and-click moving impossible (or useless). Just a few of these champions:

But hey, this is why there are other runes because runes are supposed to fit certain champions and playstyles. The old Ultimate hat didn't fit a few champs either, especially the ones with special or low-CD ults, such as:

I often don't agree with the things and buffs-nerfs they make at Riot, but this new rune seems pretty fun and sometimes game (or at least 1v1 fight) changing. I am really curious about how this will end in the game.