You are making the assumption that someone who has indulged at some point in the past few weeks or months is less productive.

That's a bad assumption to make.

People become less productive for a number of factors, some of which have different impacts on each individual.

For example, random co-worker A may be in the midst of a breakup. Their production suffers because they are sliding in to a depression and find their current life to be meaningless. They just don't really care right now, they've got other bigger fish to fry.

Random co-worker B is also going through a breakup. Turns out their partner was actually dragging down their production by nagging and constantly texting them to the point of suffocation. Now their mind is free of distractions.

Weed works the same way. For some people, they can become distracted and lose interest in activities. For others it can increase creativity and focus.

You wouldn't make relationship status a requirement for hiring someone would you? No, that would be ridiculous.

And since you are making this assumption I'm going to give you a little life tip. There are numerous people in your life who use marijuana. Friends, family, coworkers, etc. You have no idea. Because that's how little it impacts their lives, and subsequently, your life.

Don't make assumptions.