I find them to be quite useful omni-lords
No doubt. They don't show up on searches but when it comes to Dragons they fit into that weak spot on the mana curve. The
Slumbering Dragon could be an early attack deterrent and fuel for things like
Dragon Tempest but you can't really count on getting them attacking; although I guess the Sprite/Ozolith combo could get it some more counters. You're right about the other spots being terribly weak until you start looking at 4 drops where I believe you have a few choices granted you are have the easiest one of those.
Had you considered the
Dragonlord's Servant as a way to reduce the cost of those dragons?