Chip cards are more secure because they prevent cloning. Chip cards also have the magstripe for backward compatibility. In my recent experience at supermarkets in California, using the chip takes a lot longer (15+ seconds) than swiping, and can't be done in advance, while the cashier is scanning the items.
Given this inconvenience, I'd prefer to swipe my card at a POS inside a supermarket, which is monitored and under human supervision. I'd use the chip at an ATM or gas station.
Yet, some POS units (e.g. those at Trader Joe's in California) require using the chip if one is present, despite the inconvenience to the customer. Why is this?