This is highly important. No body wants someone "high as a kite" operating a vehicle. It is not in the public's best interest to have very high people driving just like having severely drunk people driving. Its all about safety.
You could frame the argument that marijuana is similar to other medicines (OTC or prescription, labeled with "do not drive or operate machinery") but the attempts to legalize marijuana for all uses - recreational - negates this point. If people can use marijuana at all times, any time, than a method of ensuring a using person is not endangering others is needed.
To sum, legalization requires a definitive method to measure "highness" to ensure safety for the public. Once this occurs and people know the rules and levels at which they can be high, then legalizing makes sense.