I'd like to see some data on the increased risks of driving while under the influence of marijuana before I lump it in with alcohol but saying that ANY reduction in focus or coordination means you are a threat to other drivers doesn't seem rational as long as you are still above a minimum threshold.
I don't hate weed. Quite the contrary! But the whole point of a smoke is to put you in a different place and that place is not conducive to the safety of those around you while steering two tons of metal at 70MPH. I'll take us off on a different and probably unpopular tangent - if you say that marijuana makes absolutely no difference to your baseline judgement, reflexes, focus, etc... then you might be hitting the pipe a bit too often (i.e., massive tolerance and habituation) and it's maybe time to consider scaling back.