I'm a collector, so I like having instruction booklets. Admittedly, I'm much more selective now then I was a few years back. Games I'm unsure about, or flat-out dislike, are generally "cartridge only". My favorite titles (Flashback, Rise of the Dragon, Soul Star, Zaxxon's Motherbase 2000, Panzer Dragoon Saga, Ghost House, etc) must be complete.
There are rare circumstances I throw my arms up and just say fuck it, like with Outlander. It was a favorite from my childhood, it was $5 and hard to find in stores. I'd like a complete copy one day, but I'm glad I have the game.
I do like having instructions though. I remember when I was little and my mom and I would Toys "R" Us, looking for games. I couldn't wait to play that shit, so I'd rip the plastic off and read the instructions. It got me hyped on the ride home. Also, they're very helpful when your internet is inconsistant and you need a hard copy to figure out how the 3-button layout works for Shadow Squadron....
That's another thing, as a collector, a hard copy appreciates in value; a file sitting on your hard drive does not. You spend damn near the same amount of money for something, yet you can't trade/sell it later if your're financially jammed up or wanna' leave something behind for your friends or family.
Digital has it's place, but I don't feel it should complete replace physical media. That's me though.