Here's an interesting blog post about a recent discussion between Neil Tyson and Richard Dawkins on biology in Science Fiction. I've heard Tyson make similar claims in the past; he certainly could be right. But after thinking about it for several years on and off, my inclination is to opt for Dawkins's take.
Biology may be undirected, but environmental impactors are likely to be similar enough even in alien environments that we should expect rough similarities between how organisms evolve, at least on average.
This depends, however, on our aliens growing up on planetary environments rather than (e.g.) in space. Is it possible for life to evolve in space? I don't think there's any way to know, but clearly any such life would have radically different variables constraining evolution than that on a planet.
The other point here that impinges on SF is one of narrative, however. It's difficult to come up with compelling narratives about creatures that are very different from humans. And the more different they are, the more difficult it becomes. It's harder to create characters with whom we can empathize if they don't think in ways similar to ourselves, if they don't act in ways we can begin to understand.
Part of that has to involve physical action: we display emotion and intention through action. An alien without roughly human-analogous appendages can't gesture. This may seem a small issue, but it's difficult to construct a decent scene where one of the main characters can't make comprehensible physical motions. And the less comprehensible they become, the worse it gets. Until it's the author who's left flailing.
That may be fine for some who want to create faceless villains or the ravening horde, but frankly one can do that with humans. Creating incomprehensible evil is actually pretty darn easy. Politicians do it all the time. So it's just as much of a cop-out to produce human-disanalogous aliens and then make them the Blob.
So at any rate while it's theoretically possible that Tyson is right and the aliens we find may all be radically different from ourselves, I think he misses the point, at least as regards SF. SF authors don't simply cook up human-like aliens because it's mindlessly easy to put a different nose on a human character. They also do so because any reasonable narrative assumes roughly human characterization and action.