Remember that if a rear derailleur fails at any respectable speed, you will have a handful of snakes in your hands until you get it shut down. For some reason I haven't quite gotten figured, if you lock up the drivetrain, your rotational momentum gets transferred around the front hub. At best, you'll be doing a nose wheelie, at worst you'll be riding your nose across the pavement (I broke a super record rear derailleur once).