If your engine vibrates a lot, that vibration will get transmitted tot he exhuast, and it may rattle. I don't have a flex section, and my downpipe goes straight to my cat with no donut gasket or anything, and it's been fine for over 2 1/2 years. My engine doesn't really vibrate that much though, since the urethane mount inserts hold it in place pretty well. I think more of my engine vibration is transmitted to the chassis thru the mounts than thru my exhaust. If your engine moves around a lot, get a flex section.