Donationware does work... see Dwarf Fortress and the Humble Bundles.
Do not mistake donationware with F2P, these are completely different beasts (althrough, I agree that some of the feeling of "helping the developer" is present in both). The problem is that when it comes to donationware there is always the same success story, it includes DF and Humble Bundle and... that's the end of the list. When it comes to F2P it's not only about Farmville, the list can go on and on, there are thousand examples (most of which most people never heard about, yet these are successful financially).
The difference between F2P and donationware is that in F2P the financial success is a norm while in donationware it is a rare exception.
At least that's the impression I'm getting, I know only about F2P, never tried donationware
There's lots of other good 'commercial' donationware too... mIRC, Sublime Text, jQuery. It's an uncommon method. Though you do have to bundle a tiny gift with it. It depends on your definition of 'donation', do TF2 hats count?
Of course, it doesn't work with a lot of things. Dwarf Fortress has to be donationware because it's permanently in alpha. mIRC is easily the most popular IRC client in the world. If they charged for it, people would just find another IRC client. And because you've got people who log onto it every day and see the nag screen, mIRC gets a trickle of money.
Look at Farmville. If people didn't drag their friends into playing, the popularity of the game would never take off. Heck, look at the failure of Farmville 2 - FV2 is a far superior game, actually very technically impressive. But without the strength of communities, it flopped. People got their friends to play. They got friends who quit to start playing again. People would click a few cows and then just toss money to not play. I mean, the funny thing is that nobody really wanted to play Farmville, and they literally paid NOT to play it. It was just a social show off thing. And when people got sick of Zynga's money grubbing, the social pillars collapsed. Nobody played because their friends were no longer playing.
It's more complicated than just running the numbers.. there's too many blurred gambles and a lot of conditions. You won't be able to sell Warcraft on donationware/F2P, but you would probably be able to pull it off with WoW.