I made an account just to reply to this.
Did you stop doing the guarantee? I don't see it on the website.
My only issues are with your 2 requirements for the guarantee, I'm somewhat unsure of the reasoning behind these other than protecting yourself financially..:
1. Fee for tutoring with guarantee versus fee for tutoring without guarantee.
You said multiple times the regular fee. I would personally not like having such a strong psychological motive against scoring well.. The money wouldn't be too big of an issue, but deciding on paying for the guarantee is similar to (in my mind) telling myself, "I am betting this amount of money that I will not score 700 or higher for 3 tries in a row."
2. Required to take the exam 3 times.
This is a big inconvenience for the test taker... I'm thinking from a personal perspective, but I wouldn't really have any incentive to take the GMAT two more times if I scored very close to 700. This requirement alone seems like a strong enough reason to prevent anyone from utilizing the guarantee. Personally, it's really the only reason I wouldn't.
Note: I've spent around $7,000 already on test prep, including private tutoring from 3 top companies. A 700 score guarantee could be a very lucrative market I bet.