But as someone who lends some of his money, I don't think that it should never be possible for lenders to lose money! As you said, if the market swings 10-15% (or more) on a natural market movement and i lose money, then i can hardly complain.
I would disagree with that: it should not, under any preventable circumstance (from shutting down the trading engine to shutting down the platform completely) be allowable to have the lenders lose money. I'm not sure how many of the people here commenting about losing money have actually been faced with the prospect of losing tens or hundreds of thousands of your hard-earned dollars. I'm not talking about BTCs that you bought 3 years ago at a fraction of the cost and you sold recently for huge profit, i'm talking about money that you earned through months and years of hard and value-creating work.
Then again, i'm sure very few people posting these comments have actually tried to convince other people to put up their money in their platform for unknown and unverified traders to play around with on margin....
Anyways, it seems we keep beating this horse after it's long dead. BFX has made it clear what their strategy is, and i'm sure most if not all the lenders (of USD as well as BTC and LTC), as well as the majority of the traders, support it.