Technically, I agree with you
I never loaned out at Poloniex, and it may in fact be more profitable than the "ridiculous interest banks offer on deposits" (though this remains to be seen). Nevertheless, you forget a very important factor, namely, that Bitfinex was doing exactly the same, and most likely they had started marginal trading (since that's what lending at an exchange comes down to) even before Poloniex. So, on the whole, I wouldn't be that fascinated at this form of passive income, and still less consider it as "much more secured"
There really is no risk free way to make even a dime out there and living itself is enough risk! When it comes to investing and especially online investments, the rule of the thumb is always to never invest whatever you cannot afford to lose, because that chance to lose it all is ever present. We do due diligence to filter out prospects that are less risky and lending at exchanges as Poloniex is one such businesses
These are mostly trite phrases which everyone knows
Something like "don't invest what you can't leave to rest". Personally, I don't want to leave to rest anything. In this way, when you are just idly holding your funds in the exchange account, you are exposing yourself to a higher risk overall than if you were trading the same amnount at the same exchange. How come? Because when you actively trade you usually trade at 2-3 exchanges so that not to lose an arbitrage opportunity and thus you have to deposit and withdraw pretty often (up to a few times daily). And when the exchange starts delaying deposits, it is a sire sign of possible future problems. In fact, when the withdrawals became a butthurt at Cryptsy, I withdrew through doges all my funds there, and I wasn't wrong. On the other hand, through wise trading you would earn more with less, therefore your total risk exposure is dramatically decreased
The only instance a lender can lose, is if the site is hacked or if the site becomes insolvent, other than those two, it is a win, win situation for lenders. The system automates the process and it is structured in such a way that defaulting is made impossible
I know how margin trading works
