First of all, to everyone who said it couldn't happen, and wasn't going to happen when I said it was possible, piss on you.
Second, it's a win. Plain and simple.
The ability for people to take both the Xbox Live Arcade, Games On-Demand, and Xbox 360 disc-based games they own and play them on the Xbox One is a win for those who own the games.
It means people can play the games that they already own on a single system, it means they can finish games they may not have finished because they couldn't afford to own both consoles and traded one in for the other, and it's a win because some teenagers became adults during the life of the Xbox One, had kids, and now want to play some of those games with their kids.
I loved the original Xbox compatibility on the Xbox 360. While there are some who never took advantage of that benefit, I did. Not only did I find the new controller better and easier to use than the original Xbox controllers (both the Duke and the S), it worked with almost all of the games I owned. The original Xbox compatibility on the Xbox 360 allowed me to trade-in my original Xbox, plus I was able to go and buy older Xbox games I never had the chance to play.
With the Xbox 360, there are games I bought and still haven't played, there are games I never completed, and there are still some games I'd like to play yet. Games I bought through Games on Demand, and Xbox Live Arcade. It's a win.
Backward compatibility gives gamers options, regardless of what system it's on. It's a win.