You'll burn up the clutch and pressure plate by riding it for so long. The clutch is designed to be pushed in, then released in one smooth motion. It'll can get hot and burn up if you don't. Going through each gear (downshifting smoothly) also saves your brakes (brake pads and drums/disks), riding your brakes and the clutch will leave you calling for a ride. And both cost a lot to repair (time and money).