I think that while you're right about the pragmatic motivations for portraying aliens as humans in popular entertainment, you're wrong to dismiss the idea of convergent evolution. At least, not in a "weak" form; the "strong" form of identical human aliens reeks of mysticism and superstition.

I forget which of so many Richard Dawkins books I read in which he tackled the question of how many limbs is the "right" number. It's not arbitrary, it turns out, not just a matter of random mutations that either work or don't. Each limb an organism grows has a cost, in energy and material, to create and maintain it. And there's increased complexity of its nervous system and CPU to coordinate multiple limbs.

Here on Earth, evolution has produced only two answers, four and five limbs (if you don't count segmented organisms such as centipedes). Four limbs tends to come in a bilateral symmetry design, five limbs with radial symmetry. Horses and starfish.

Why not three? Dawkins thought it was suboptimum, because loss of one limb would likely lead to the death of an individual, while four limbs minus one is survivable, with a limp. (Bipedalism has to be learned over many generations; a three-legger can't suddenly convert.)

Why not six or more? Diminishing returns.

These ideas about terrestrial evolution can, I think, be legitimately generalized to consider life elsewhere. If common physical factors drive a general trend toward a multicellular, chordate, design, then I'd expect that planet to have lots of four-legged critters. And if one evolves into a tool-user, it's going to use the two spare limbs and get by with bipedalism.

I'd generalize, too, that the five-limbed design will be "popular" on a minority of life-bearing worlds. The problem with pentacularism when it comes to evolving a tool-user is that five minus two equals three, and tripedalism wouldn't produce a smooth efficient gate, thus creating a gantlet a species would have to run to free up manipulators; but the more natural quadrupedalism leaves only one spare limb for tool-use.

I should reiterate that I'm not dubious about the non-quadripedal->bipedal scenarios because they're "weird", but because they seem to offer suboptimum energy and material use.

So maybe there's a real reason for thinking that we might encounter sort-of-humanoid shaped aliens.