That's not necessarily true, it all depends on what your desired outcome is. Stepper systems can be open loop as long as you're aware of what the torque threshold is and make sure you don't exceed that. Obviously a crash would exceed that, but there's ways to detect that (e.g., stallguard on Trinamics stuff) without adding an encoder. The whole point of open-loop stepper systems is that you make sure you stay under the torque limit by a margin (say, 30% or so) and you are fine. A properly designed system should never lose steps ever, besides in a crash event. Or if you load it up with too much side force, but you can get that on any machine.