You initial thought about spinning gears having lower gear ratios is usually true, but what you do not realize is that you cannot equate this supposed low gear ratio to those from casting reels.
Spools on spinning reels can be big and as a result, they can recover more line than a typical casting reel despite their "lower" gear ratio.
Take a look at the 2500 Symetre FJ. It a gear ratio of 6.2:1 which is lower that the typical "burner" casting reel, yet it recovers 35" of line, which is more than a burner reel.
I like to use both setups for cranks. If the casting reel can toss it, I'd probably use it because of the lower line recovery rate, but on the other side of the coin, spinning rigs cast lighter cranks more easily.
Bottom line I think it boils down to personal preference. Cranking with a spinning rig feels more comfortable to me, but I do use both.
That's what I like about BPS's website they list the recovery rate for most reels. I just got a new KVD baitcaster for cranking that has a 5.1.1 ratio the brings in 21inches per crank. It was nice to be able to compare it to other reels and ratio's. I also have a crazy theory about spinning reels. With spinning reels I often don't have a very steady retrieve it sputters in a strange erratic action. This action seemed to give me more reaction strikes on buzz frogs and other baits. I prefer the feel of baitcasting equipment and don't use spinning reels often but I guess it's my new year resolution to try and use the spinning set up more often. Also Hunt4Hawgs if your going to throw some large cranks a baitcaster is the way to go.