Anything at or above 15% is completely non-sensical, IMO. If you sell, the long term cap gains are that amount—which you pay once. Why sign up to pay cap gains yearly?
Actually, MORE than cap gains yearly, because the cap gains are only on the appreciation, whereas the loan is based on the full amount loaned.
If dollar inflation is > 15% over the term of the loan you win plus you get the collateral back, including btc price appreciation. It's a good deal if you don't get rugged.
Risk-adjusted...maybe not such a good deal. But that's the rub. How do you really know what the risk is?
The reason to do it is that you think bitcoin will appreciate well above 15% in the coming year and you want to do whatever you can to keep what you have.