The problem with cloud drake is that, unless you're roaming, it's effectively useless. It's only non-combat movement; so while the other drakes give direct damage, sustain, or objective control, the air drake's effect is more or less... useless? come later game where you're constantly poking or dancing around for a teamfight- or even farming.
Spicy and Rack-Salid Drakes effects are always in effect
Refreshing drake only when avoiding champs/towers
Vroom Drake is only ONLY active when nobody is hitting you and you're hitting nobody.
In essence, it has the lowest "uptime" and smallest noticeable effect, that's why people generally ignore it; it's underwhelming.
oh, and as for the Refreshing drake, the restoration is... well it's goddamn fantastic. Throw skills till you're about at 1/3rd mana, then simply cast them when you have excess mana, it restores it fast enough that you, effectively can act as if you don't have mana on some champions. Really nice for lane sustain to about half-HP as well if you take some poke yourself.