Well, here... OP, what does an alien look to you? I mean, sure, I have a vague idea of what an alien looks like... It's got a big head, big black eyes, and is grey to green. And that's just a maybe, if you believe in UFO's.
An alien can take an number of appearances from the standard biped to the crazy amorphous blob. I will admit to a degree of favoritism concerning non-humanoids simply due to the exotic (non-human) appearance. And I will say that of the non-humanoid aliens the inscetoid ones would have to be my favorite, mainly because our own insects are so prevalent and industrious; ants form 15–25% of the terrestrial animal biomass on the planet; and the thought of taking that concept and applying it to a larger, more intelligent, space faring species is interesting to me.
What I think you're upset about is the indifference the world shows to people. Stay with me here. Some people can't do certain things, so they suck at them. Or better yet, are lesser. That's a part of the beauty of this game. Because it shows that some people can't do certain things.
If I had to sum up why I want to see aliens like the Rachni and Elcor to take on more prominent roles in future titles in one sentence I would have to say:
"Humans are boring"
Nothing against BioWare's characters, or the political/cultural/religious issues they use said characters to tackle, but I can turn on the news at any time and see that 24/7 in the real world. Mass Effect is a science fiction setting, I want to go out and discover the people, places, things, etc. that are different from us, to see the truly alien side of the universe. I enjoy the human characters and elements in the setting don't get me wrong; but there comes a point when it seems that all we are getting is the human side and all of the interesting aliens are being shoved into the background. I know that aliens like the Elcor and Hanar are unable to the exact things that bipeds can do, and having diversity in the galaxy is a good thing, but relegating them to the sidelines, or worse; in the Elcor's and Hanar's case; turning them into the joke races because they can't do the things we can, or they look different, or talk funny, etc. just makes the universe feel shallow and very human centric.
- An Elcor might not be as adept at communication or as quick as a human, but they can rip a person in half with their bare hands, they can carry the equivalent of tank cannons on their backs as part of their standard combat equipment, and they can take a heck of lot more punishment than a human can before falling.
- A Rachni soldier can't utilize human weapons and armor, or blend into a crowd easily but they can mature in a matter of weeks, and they can engage targets using mass effect weapons and kinetic shields using nothing but their claws and acidic spit, and they can survive in hostile environments (like a vacuum or corrosive atmosphere) without a space suit.
And those are just the combat oriented aspects that aliens like the Elcor and Rachni bring to the table. They have other advantages and disadvantages compared to humanity, but are kept in the background because they can't do 100% of the things that humans can, forgetting that many of these aliens would actually excel in areas that humans would be hard pressed or unable to do.
But that's no reason that their lives don't matter as much as ours. In every single game (I think), Shepard said, "I'm fighting for everybody in the galaxy!" He didn't say, "I'm fighting for every biped in galaxy!" You want an alien who looks like an alien (in other words, without being a bipedal creature) to do something that everybody else can do. It's possible, I just don't think it's probable.
Shepard was fighting to save the galaxy it's true, but s/he was more than willing to throw other races under the bus to save Earth, even though it; until the last bit of ME 3; was tactfully unimportant. Even more telling is that the more human aliens are afforded more worth in the eyes of galaxy than the non-human ones:
- Sabotage the Genophage? You monster! vs. Exterminated the Rachni? Great job Shepard!
or
- Sided with the Geth? Your a murderer Shepard! vs. Sided with the Quarians? Well done Shepard!
In those two scenarios the Rachi and Geth would be the better choice, militarily speaking, since the Rachni can breed combat capable solders in a matter of weeks instead of years for the Krogan, all Rachni have a built in QEC, Rachni have their own natural weapons and armor on par with the galaxy's weapons and defenses, and Rachni are far more disciplined than the warlike Krogan. Likewise the Geth don't need to eat, or sleep, or breath an atmosphere, they aren't vulnerable to fatal infection from contact with the common cold, and their weapons and ships are built to spec instead of being second hand purchases like the Quarians.
Obviously, having all four species help in the fight against the Reapers would be better simply due to numbers and diverse tactics employed by each species, but the narrative places and implied sense of worth on the races that are more like us.