The problem with non-bipedal aliens is that players want to be able to use the enemy's weapons and possibly armor as well. Keeping the aliens relatively humanoid means that weapon use will stay pretty compatible. So far, it looks like the Cabal may be the only alien that uses weapons we can't (due to their apparent size). Now, I realize having multiple legs doesn't change how many arms an alien has, but once it gets to a point, it starts to get a bit ridiculous (6 legs and 2 arms). Things get to a point where it just becomes inefficient, biologically, and our bipedal form is remarkably efficient energy-wise. Chances are, a lifeform able to survive in the same environment that we are will have had similar factors present in its evolutionary path. It also comes back to story-telling. Humans identify with humanoid aliens much better than they do with crazy and outlandish shapes (it's the same reason so many stories revolving around animals give them humanoid postures and such (like Redwall and Narnia). A blob of goo isn't very relatable to us on an emotional and narrative scale, and neither is an alien with 8 arms and 3 legs. Destiny is promising a story we can get engrossed in, and that is nearly impossible to do with non-humanoid shapes (unless there is extensive text and thought conveyance from them, which won't really happen in a game). Animation is also very difficult for non-bipedal characters. The only reason our animation is so believable now is because of motion capture, and there aren't a large number of six-legged people available for mocap on non-bipedal characters. Some animation is still done by hand, but that requires a lot more in regards to time and resources, and still ends up being questionable whether it will work well. Mocap is pretty much guaranteed to look natural and realistic because it's following natural movement. To animate a non-human shape would take much longer, would require more manpower, and would probably end up with less animation than motion capture. Until those hurdles are crossed, non-bipedal aliens will be limited mostly to books, and as non-relatable antagonists.