The point is that your mindset going in to the trip needs to be pristine. If you're in a bad place, stressed out, or worried about anything, you WILL think about it during your trip, which can take you to a really dark place. It's hard to explain the horrors of a bad trip if you've never had one.

Also, going back to work on Monday after having your mind blown isn't the same as coming in to work with an alcohol hangover. You may not be able to function as a normal human for a couple days after a heavy trip.