Systems that use prefix syntax must enter a mode to wait for the user to specify the object. Keeping track of mode changes can distract a user from the task at hand.
Users often get “stuck” in a mode. An oft-heard question is, “How do I get out of this mode?”
If unmodified letter keys are typed in command mode but the user thinks the system is in text mode, unintended and sometimes disastrous results ensue.