I have always assumed the purpose of the Keep Plates is to stop the carriage falling off, not to hold it down to the bed. In normal use the carriage is held down by the cutting forces. Most larger lathes I've come across work like this. If you have problems with slack in the system when facing off, tighten the clamp on the carriage. That's what its for. In general, any axis that is not moving when making a cut should be locked.
