The problem is the potential for zombie-fun outside of the default configurations. Defaults can't really stand up against that.
Are you guys kidding me... obviously you haven't even given the default maps a chance for infection. Opens your minds a little... you don't need a specially forged map to play infection. A lot of the forged infection maps involve super-camping bases where it is almost impossible for zombies to infect someone. Sometimes you need to just go with the default maps. I that a lot of maps aren't really suitable for infection, but maps such as High Ground, Last Resort and Sandtrap are great for infection. My favorite way to play infection is on default High Ground where the zombies have no shields, 300% speed, humans spawn with shotguns and pistols limited ammo. My friends and I have endless amounts of fun playing this. Try it sometime, you'll learn to love it.