No I don't want active pot/cocaine/narcotics user's in the military, but Matt is right. Past use should not disqualify someone to serve as long as they agree to regular testing and a commitment to refrain from using it going forward. Many employers drug test their employee's for the same reason and use the tests to prevent hiring active drug users or fire one's that don't think it matters to the company. Now if they are current users and cannot pass a drug test, then they are not ready for a serious commitment yet. I don't know how far back the drug tests go in finding past usage but they should be able to pass the tests prior to entry, and submit to regular checks while in service until they can prove they can be trusted at the very least to take care of themselves. That's my opinion anyways.
Used to be a lot more smokers and now we have the vapers. No where near as many smokers as their used to be. Smoking and PT just don't go together very well.