It is "safe" for the most part, especially on Bitfinex due to their size and track record. Lending for margin on exchanges has proven to be a stabe return for some investors and is geared more towards the risk averse who would rather not take a chance in playing with the markets but rather have the opportunity to generate slight gains regardless of whether the market is going up or down.

I would not expect to get rich on margin lending/swaps though.