AltruEgo: The game is a purely multiplayer game (with the exception of tutorial levels) but very little else is set in stone.
And right there is where it fails for me.
First of all, I don't like to be dependent on others in order to play a game I bought.
Secondly, a multiplayer-only game has to be quite successful in order to make it at all. The chances of that happening with a self-funded indie game from a totally unknown three-man team are minuscule. This means that if you sell less than, say, 20,000 copies, then all those people will have wasted their money, because the game will be dead. Look at how many multiplayer-only mods have been made for various games, and how many of them have failed completely, because the player base just wasn't there.
It's a vicious circle. A small player base means that games are hard to find, especially without a lot of widespread hype beforehand, to ensure that lots of copies will be bought on release. When games are hard to find, most players will soon look elsewhere for their gaming fix, making games even harder to find. And so, the game dies.
I understand why small devs prefer to make multiplayer-only games. It means you don't have to code all the really difficult bits, such as enemy AI, and spend time on making a campaign structure. Just understand that it's a gamble, and that chances are you'll lose. And so will your customers.