As a matter of fact, this is not true. Laboratory tests with road cyclists show virtually no difference in efficiency between platform and clipless pedals. Clipless - or clips and straps - are about foot retention, not pedalling efficiency.
I've toured many thousands of miles on platform pedals. It helps to minimise the load because I don't need to carry spare shoes. If you're worried about foot retention, get BMX - style pedals, the big platform and the studs make it very easy to maintain your foot in the right position.