Comparing it to PS3/Xbox/Wii U is missing the point.
You don't. You put on your eyepatch, grab your cutlass, and raise the Jolly Roger.
There's pretty much zero downside.
People don't pirate 360 games because it's probably fairly annoying - you need a 360 console to begin with, then mod it which might need an older one running an out-of-date OS, and likely can't play multiplayer without the risk of your account being banned. Plus the size of games means torrents which means letters to your ISP. And if it's a couple years old you can probably buy the game for $10-20.
Pirating games for older consoles is trivial in comparison - any computer you have and most smartphones can emulate a PS1 or older almost perfectly. And the game sizes are tiny.
If you were asking about a moral justification, there's always the "it's not being sold new, so the makers won't get my money if I buy it used anyway." Though by now at least 95% of games that people actually play have been re-released, so that justification doesn't hold much water.
Anyway, he could perhaps rip ROMs himself if he really wanted to bother.
Even ripping your own, legality is still probably a bit of a grey area.
Sometimes my girlfriend may want to play a SNES game while I'm using my laptop.
Sometimes I want to reinstall my laptop and don't want that to impact my gaming console.
Sometimes I may not want to bother with having to plug in my laptop in the TV and find the controller, to play 20 minutes of Dr. Mario. (I'm very lazy)
I'd rather have several cheap, interchangeable devices rather than everything from a single one :) (that applies to my laptop too; I did all of my programming and work from a 300$ netbook for a couple years, that was nice and I wasn't too heartbroken when my gf at the time spilled water on it and killed it)
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It's something that slots in alongside other consoles rather than replacing or even competing with them.