I'm a month late to the party, but I'm new to DW and learning, too, so I thought I'd give my take on the question.  But the first thing I thought was...

Oh, hell yeah!  This is rife with storytelling possibilities!   They're elementals!  The most powerful force in the world (or so they think!).  And this puny human thinks he can just conjure them up and give them orders?  As if! 

I admit that when I first read your question, I immediately thought as you did -- "why would you ever choose to not have the effect?" *

But I think that the rule is saying that calling on the elemental powers is dangerous and unpredictable.  And if you play it that way then you will see players choosing to abort the effect they want just so that they can avoid the consequences.  Really, they are going to be hoping to roll 10+ in order for the thing to work and avoid having to pay one of the consequences.  If they don't get that 10+, then they can abort altogether or take the consequences.  The player's choice is going to tell you a lot about what they want from you in terms of the adventure.

In other words, play the elemental like an office temp with a bad attitude and he's being forced to work late.  On prom night.  Without any supervision.   And the player is the owner of that business.

Think of how terrifying "paying the price" and "retaining control" just became.  To continue the metaphor, I'll riff off Office Space. On a 10+ you get the TPS report out on time but the company's funds are embezzled.  So then the choice for the player is -- exactly how badly do you want this report?  And if they only get the 7-9 -- maybe they say the report just isn't worth it.  Maybe it is.  Either way, it's advancing the story.

(Just out of curiosity - why were you thinking of limiting it to harming the caster?  If fire is "out of control" then personally I wouldn't see it discriminating.  But even with that stipulation - is harming something or someone the caster cares about harming him?)

So let's take the 16 HP Dragon scenario and see what happens with an Air example:

The effect desired is to down-draft the dragon and force it to land so that the rest of the party can attack... 
Paying the price - ...and the tremendous wind also flattens the nearby town.  The villagers might be grateful the dragon is gone but not so happy about the damage.
Paying the price - ...and the wind blows sand into the eyes of the party and blinds them.
Out of control - ...and the wind flames the buildings set on fire by the dragon and a firestorm erupts, burning the town to the ground.     
Out of control -- ...and the party is also tossed about for damage

And you can certainly go the "debt" route as well -- if the druid hasn't paid the debt, then maybe the elemental doesn't show up at all -- or maybe it demands a much higher price.

If you play those elementals to the hilt, I think you're going to have a lot of fun.  If the other player's aren't shouting "for the love of god, no more elemental magic" then push harder.  :-)
(In other words, I would treat them like demons in Sorcerer for those who might have played that game)

Best,
Nick

* I do kind of agree about what you said about the stakes, though.  It seems kind of weak.  And I'd certainly be tempted to tweak it by just dropping the "and you get what you want".  So a 10+ has no real consequences, but the 7-9 certainly does.