At the moment, because it is all we have to study, DNA-based life (and its RNA-based cousins) seems the only viable solution. So far we have not come upon anything we would call "living" that isn't based on some form of the nucleic acids. I think that until we meet such an entity, we will have trouble imagining any other kind of life. But heck, we have such a wild diversity of shapes and sizes based on DNA that who's to say we haven't already seen the shape of aliens? We have snakes with no external limbs, starfish with five-fold symmetry, arachnids with eight arms, insects with a carapace and claws, and these pesky humans.
But Jon, I wasn't actually against panspermia. My comments may have come across stronger than I intended. Don't apologize for bringing it up. If it is true, then it actually increases the odds that if we DO ever meet other intelligent races, we will have more in common than we would have ever imagined. In a way that is comforting. In a way that is very frightening. A race that can bring forth Gandhi, Mother Teresa, and Martin Luther King, but can also bring forth Temujin (Genghis Khan), Hitler, and Torquemada (of the Spanish Inquisition)? There is no telling where WE are going and thus no telling where any other race could go.