Hi JWG
My previous answer was really meant flippantly, since I can't write a serious response while rewatching CI

My fingers always wander off key and it comes out in gibberish.
Firstly, JaredD is wise. Love 'em or hate 'em, any character that challenges you is worth more as a piece of art than another feel-good character that makes you giggle. A lit/tv prof of mine always says that art is something that 'contains truth.' Jinx and Squish, for better or worse, exist in the world.
But your argument makes a couple of substantial points which have come up a bit before, and will certainly come up again, and I'd like to address those.
The first is a personal distaste for the characters. I'm sorry, it's not pleasant when that happens. I know- I watched JAG for -years- and from the very beginning I absolutely loathed Harm and Mac, the lead characters. Loathed them, and they only got worse as time went on. But I adored the entire supporting cast, so I made do. At some point I became disgusted with the actors as well as the characters, which is where being an audience gets tricky - and it sounds like this is part of the problem you're having with Jinx. It's unfair to judge her performance style in relation to her 'typecastness,' given that she was cast to fill a specific role in the show. I have little prior experience with her, and find it refreshing to see an older female character who is neither a dotingly loving mother, nor any sort of poster child for feminism.
Unfortunately, nothing can be done about this problem, except discussion about who and why they are, which might help you to appreciate them. Or they might grow on you: the series is still just beginning, and the writers seem good enough to deserve a little faith that they'll make it work. Or you can ignore them, since they aren't on all that much.
A second argument you brought up is that the family storylines interupt the flow of the primary plot. And it does. It really really does. It would be smoother if parallels in the stories were more pronounced, but honestly when that starts to happen, unless it's done really well, I find it a little condescending and overwritten. At any rate, it's a format choice of the producers/writers to have an A-Plot and a B-Plot, and all that's left is to consider what purpose it's serving. Such as: how are they shaping the A-Plot around it? Commercials are important to many shows, since they allow for brief cliffhangers and moments of extended dramatic tension, and scene changes can do the same thing.
One little note more: I disagree with the postulation that Jinx and Squish are there to show where Mary came from, or even how she interacts with people she loves (though doesn't respect.) (Which is important if considered in conjuction with her lately realized respect for Marshall.) They're characters in their own right, albeit underdeveloped at this point. Squish is the little sis eh? So Mary had as big an influence on her growing up as Jinx did. And we're starting to see a very interesting relationship between Jinx and Squish- Squish plays a big sister role, with similar firmness to Mary, though less high brow and condescending.
I don't know where you fall demographic and audience-wise, but your eloquent rant (we've all made them, though mine tend to come out with more bleeped bits) indicates you're a thinker. I hope you find a way to engage with the elements of the show you dislike, whether by coming here to rant and disect them, or even not paying them attention. Because there is so much awesomeness, and it would be way harsh if the B-Plots killed the rest of the show for you.