Q:

Why are comment lists abridged in the middle?

In longish lists of comments the list gets abridged and a "more comments" link shows up at the end, kind of like an ellipsis. Normally, one would expect that such a list would be trimmed at the end, but on English.SE (and possibly on other SE sites) the elisions can occur anywhere in the list.
For an ordinary, non-sequential list, that would not be a problem:

apple
orange
peach
pear
plum

could safely be abridged to

apple
orange
plum
[more]

But comment lists involve questions and answers. And it often happens that items appear in the abridged list which are answers to questions that have been hidden. This is confusing, to say the least. Consider:
@mike: Do you mean an avocado is a fruit? -ben
@ben: No, it is a vegetable. -mike
@mike: Really? Did you look it up in the dictionary? -ben
@ben: I looked it up on Wikipedia. -mike
@mike: I don't think Wikipedia is a dictionary. - ben
@ben: Trust me, it is a dictionary. -mike

If we remove comments from the middle we could wind up with something like
@mike: Do you mean an avocado is a fruit? -ben
@ben: Trust me, it is a dictionary. -mike
[more comments]

Is that what we want? 

A:

Robusto has explained the problem. I propose that a clearly visible button "expand contracted comments" be put either right above the first comment or at the first place chronologically where truncation has taken its toll.
[Edit] I also propose that it be a bit more prominent, preferably in a different colour.
[Edit2]
We should all try these beautiful Greasemonkey scripts, by one of our own moderators:

https://meta.stackexchange.com/questions/43055/greasemonkey-script-for-displaying-threaded-comments/76235#76235
https://meta.stackexchange.com/questions/38600/script-to-add-reply-links-to-comments

A:

Please see Meta StackOverflow for an answer to this question. 
Short version: it is what it is and SE is fine with that.

A:

I agree that this is sometimes a problem, but all the obvious alternatives have problems too, and on the whole I’d agree with Jeff that this is the least worst option I’ve seen suggested.
Show all comments?  Far too cluttering, and steals focus from actual answers; this is pretty clearly an option that Jeff (& co.) considered at the beginning and definitively rejected, for the sake of making SO/SE the system they are.
Abridge after the first, say, 6 comments?  Then misleading comment #6
@pyramus: Actually, avocados are a kind of bean.  See for instance [authoritative source].

remains visible, while the rebuttal #7
@thisbe: Uh, you just made that up completely. That source describes them as a fruit.

remains hidden however many upvotes it gets.  This is a problem that already happens sometimes if the rebuttal doesn’t get enough/any upvotes, and would be really horrible if there were no way to redress it with upvotes!
Put the expand button at the first point of abridgement? (Cerberus’ suggestion above)  This has some different problems.  Would there be buttons at every point of abridgement?  (If so: horrible clutter/redundancy!  If not, then your original problem can still occur at later such points.)  Would there always be a button at the bottom?  (If so: redundancy again!  If not, it’s slightly confusing that it’ll be at a different place at the list each time.)
Subtly change the comment separator (eg from single to double line) at points of abridgement)?  I just thought of this myself so can’t see any problems with it… except that perhaps it’s too subtle to really solve @Robusto’s problem much?  Consider finding problems with this solution as an exercise for the reader :-)

