ALRIGHT I'M BACK AT 4 AM SLEEP IS FOR THE WEAK
@Sshodan
nitpicking wall of text ahoy
Logic in you argument is flawed.
Let's follow:
Premise 1. There should be other plot characters except Shepard
Premise 2. Equal number of plot characters on both sides
Premise 3. Plot characters' arcs should be related to the protagonist's arc
Premise 4. Number of plot characters should be about 4-6
Conclusion 1. We should have a plot character in our squad - valid, from P1 and P3.
Conclusion 2. We shouldn't make all plot characters into squadmates - valid, from P2 and P4
Conclusion 3. We shouldn't make all squadmates into plot characters - valid, from P2 and P4
note: in above statements the "all" constraint taken verbatim from your post
Conclusion 4. "Having one plot char is a good compromise" - ...insufficient data.
Having
some (for ex.,3 or hell, at least 2) of plot characters as squadmates and by extension having some squadmates as plot characters is a "compromise" (from C1-C3).
I fully accept your premises (well, P2 is somewhat arguable, but whatever). And while I
could accept C4 on its own merits, it doesn't really follow from you argument

Now, if I take into account another your premise which is based on rather subjective motivation - "yet you do not have to tolerate more than one char that can't be properly ignored." (subjective because it's not based on logic and/or at least common sense, since 1) many people may like the character in question and don't want to ignore him/her, 2) other people tend not to like anything "forced" on them, even if it's only one character, thus conclusion based on this premise will not be universally true), it makes more sense.
But it's not true that that character would be the only one we can't ignore. It wouldn't be even the only
squadmate we can't ignore (mandatory squadmates, anyone?)
And honestly, I'd rather have 3 characters have high plot relevance because the chances that I will like at least one one of them are much higher than . If we have only one character with high plot relevance, and the player doesn't like him/her, he's stuck with this character
all the time, but with N, N>1, characters the player can sometimes take a break from someone who annoys the crap out of him.
Another argument would be the ratio of plot
points to plot
characters, because really, even if we accept that Liara is
the plot-relevant squadmate, some of the things she had done and was involved with could be attributed to other
non-squadmate plot characters.
/end nerdwank. Yeah, I'm just really bored
You see, I can make a stand and ask: "Why Liara? Why not Tali? /*and for the record: I dislike Tali and Liara equally*/ Or Wrex? Or whoever? What's so special about Liara's
personality /*not talking about her character arc here*/ that makes her the perfect plot advancement device?" But that would be a bit silly, of course. It's
obvious why it's Liara *cough*seepostsabove*cough*
There are two things bother me personally most about Liara (aside from the whole "Second most powerful person in the Galaxy" thing - it seems, we won't agree on that):
1) It's old news, but her personality shift between ME1 and ME2. Before anyone says anything, I know it's
justifiable, but it's justifiable post-factum. Nothing in ME1 suggests her development in that way. We just get a 2-year gap with giant neon sign "insert character development here". Let's do the plot-twist dance! It just seems so...arbitrary.
I'm sure Liara fans will disagree, and I actually welcome any comments on this point, because this is something I genuinely don't understand
2) Why is she plot-friends with Shepard? Regardless of his attitude towards her in ME1? It's one thing to put a character into MarySue-ish role and shove her into players face for "story reasons", but it's whole another matter when you are actually told how you protagonist should feel in a
roleplaying game. It's never a good thing.
//insert obligatory apologetic "English isn't my first language" comment here, I can't write sh*t