There's a lot of evidence ( read the forums on Dexterity ) that giving away too much harms sales. So it's not quite as easy as giving more away. You really have to balance it out and that will vary according to the game.
On the whole, I think you're probably right that people ( myself included ) are erring on the side of caution. Best Friends is one of the best selling Blitz games I know of, and Mike took the Doom approach and gave away 1/3 of all levels + most obstacles. But even then, it's difficult to know whether he would have sold even more if he had given less away. I too was a little disappointed that there wasn't more to the full version of Best Friends, and although that doesn't harm sales, it does make you think twice about buying something else from the same people. That also has to be considered.
On the whole, I think you're probably right that people ( myself included ) are erring on the side of caution. Best Friends is one of the best selling Blitz games I know of, and Mike took the Doom approach and gave away 1/3 of all levels + most obstacles. But even then, it's difficult to know whether he would have sold even more if he had given less away. I too was a little disappointed that there wasn't more to the full version of Best Friends, and although that doesn't harm sales, it does make you think twice about buying something else from the same people. That also has to be considered.