Legal? Only if we are going to have implied-consent blood/urine test laws on top of breath testing (Turn down the test, lose your license). I don't really want to go there.
I personally think actually legalizing it, as opposed to just reducing it to a low-level infraction rather than a serious offense, is one step too far, and if it is legalized, a couple of years down the road productivity for an awful lot of the workforce is going to look like we hired a bunch of French Teamsters who immediately went on a work slow-down and/or got Alzheimer's, then started phoning every sandwich and pizza delivery joint within driving range.