I'm so glad I was turned off Star Fox 64 3D and Mario Tennis because of the demos(IMO they are 'meh' games which deserve bargain bin treatment), probably would have bought them otherwise and regretted it later. But yeah thinking about it more, I'm probably the minority.
Okay, Starfox 64 3D?
You're not getting the full experience from a demo, you have no idea what you're actually missing from it.
It's an arcade experience and the fun comes from all the various different paths you can take through the game, and improving your score with each run.
Mario Tennis I get because I tried it, and was very underwhelmed.