As I see it Sauber got their only points last year due to reliability and ending up in the points, it's really their only chance this year or next year either. Getting a slower and less reliable engine that I simply don't think has even an outside shot at being competitive till 2019, seems like a truly awful decision.
My guess would be they were only offered a one year deal for 2016 Ferrari engines because Ferrari knew they were moving to Honda and were unwilling to offer a 2017 engine for Honda to get ideas and data from through Sauber. Probably a case of Sauber could have current Ferrari engines if and only if they sign a 3 year engine deal.
Saving some cash, but giving up the reliability they desperately need to get some lucky points which are crucial to getting some coverage and getting more sponsorship cash, seems like a bad deal. They have also shown to do pretty poorly in making a car in recent years, meaning a better engine probably offers better performance gain per £ spent.
It's great for Honda, they've needed the extra mileage/running since the start, but it seems like nothing but downsides to Sauber. Unless Honda are once again signing up to an insane deal, free engines plus 10-20mil sponsorship and paying towards drivers.... in which I can understand Sauber going for it but I can't imagine Honda signing up to another utterly one sided deal.