Not only does the lockring not necessarily stay in place on a hard stop, but when it spins off, your chain derails and everything jams somewhere in your rear hub, chainstays, or cranks. At best you damage your bike seriously; at worst, you get thrown off the bike and you kiss asphalt (or a windshield). I wouldn't do it on a track, much less on the road. There are many people who ride track without a lockring, but that's a very different scene and even then I don't like the derailment risk. It's basically laziness not to use it, so why?