They should have online co-op as an option for Super Mario 3D World, More people would have bought it I think. Local co-op is great too but not ideal for older games or people who don't live near their friends. I love Nintendo but this stuff is continually disappointing. Just like New Super Mario Bros U(or any of those games) and Mario Party for the 3DS didn't have online. Bad choices. But I appreciate the online in Mario Kart 7, Luigi's Mansion: Dark Moon, Kid Icarus Uprising, etc.

Umm... just because they excluded online? 

OT: I don't give to craps to be honest. But I know other people do, and diminishing options is never good. However, I do really like the ghost players that you go against on your second round of a level. A nice touch.

If the reasons he gives are true, then yes. Poor/Lazy game design.