There is a ton of people in this thread disagreeing with the "Rule of Fifteens".
Sure, you may find the coffee drinkable for more than 2 weeks from the roast date, but can you say it's as good as when it was fresh?
Properly roasted high quality coffee undergoes significant change in those first two weeks as well, and the flavour profile can change completely from one week to the next, especially in espresso.
After two weeks, give or take a few days, the coffee starts smelling a bit "stale", it blooms less, it's just not as good as it used to be. Sure, you can drink it, and there's nothing wrong with that, but if you wanted to experience it at it's peak, you should have drank it already.
Why would roasters, baristas, and nearly everyone in the coffee industry advocate getting fresh coffee? It's way easier for a roaster to ship you a package once per month, or even once every two months, but they would like you to experience their coffee at its best.
When it comes to ground coffee, I think that everything should be ground as close to the brewing as possible, as the differences are immense. The same point stands as before, why would baristas freshly grind the coffee for each and every cup; it's much easier to pregrind a large amount in the morning that's going to last you an entire day (and a lot of places with very bad coffee do this, at least here where I'm from).
I can't comment on the 15 months rule for green coffee because I don't roast (yet). Edit: This is all coming from my personal experience, and if you feel you don't taste the difference between fresh and slightly less fresh (whatever that may mean for you exactly) coffee, then that's fine as well. But some people do, and that's why they keep recommending it to others.