*I wonder if we have systems without yast2-storage required.*

No idea, Jiri should know better, but IMHO we should allow it.

If you build an appliance in Studio you very likely won't have any Windows partition there and thus the big set of required packages is just a wasted space... (Think also about Webyast based appliances, where yast2-storage in likely not needed as the partitioning is done by Studio itself.)