The dangerous thing is having a tire on the back of your car lose traction while the front keeps gripping, which will cause your car to fishtail or even spin. This is why you want to but the tires with the best traction on the rear, but the trouble is that if you’re comparing two different tread patterns you don’t really know which one actually does have the best traction. Or one tread pattern may be better in one situation and the other one in another. Having all four of them matched in tire design and treadwear is the only way to get reasonably predictable handling.
That said, on pavement during the summer it really shouldn’t be that much of an issue. I wouldn’t risk it in winter, though.