Can any TeX experts explain to me why delimiters don't resize by default? Why do we need to insert so many \left and \right commands?
Edit: Here is what Knuth says (p 148 in my copy)
At this point you are probably wondering why you should bother learning about |\bigl| and |\bigr| and their relatives, when |\left| and |\right| are there to calculate sizes for you automatically. Well, it's true that |\left| and |\right| are quite handy, but there are at least three situations in which you will want to use your own wisdom when selecting the proper delimiter size: \ (1)~Sometimes |\left| and |\right| choose a smaller delimiter than you want. For example, we used |\bigl| and |\bigr| to produce $\bigl\vert\vert x\vert- \vert y\vert\bigr\vert$ in one of the previous illustrations; |\left| and |\right| don't make things any bigger than necessary, so `|$\left|\||\left|\||x\right|\||-\left|\||y\right|\||\right|\||$|' yields only `$\left\vert \left\vert x\right\vert -\left\vert y\right\vert \right\vert$'. \ (2)~Sometimes |\left| and |\right| choose a larger delimiter than you want. This happens most frequently when they enclose a large operator in a display; for example, compare the following two formulas: \beginlongdisplaymathdemo \noalign{\vskip 6pt} |$$\left( \sum_{k=1}^n A_k \right)$$|&\left( \sum_{k=1}^n A_k \right)\cr \noalign{\vskip 3pt} |$$\biggl( \sum_{k=1}^n A_k \biggr)$$|&\biggl( \sum_{k=1}^n A_k \biggr)\cr \endmathdemo The rules of\/ |\left| and |\right| cause them to enclose the ^|\sum| together with its ^{limits}, but in special cases like this it looks better to let the limits hang out a~bit; |\bigg| delimiters are better here. \ (3)~Sometimes you need to break a huge displayed formula into two or more separate lines, and you want to make sure that its opening and closing delimiters have the same size; but you can't use |\left| on the first line and |\right| on the last, since |\left| and |\right| must occur in pairs. The solution is to use |\Biggl| (say) on the first line and |\Biggr| on the last.
... delimiters adapt their size to the material enclosed, rendering \left and \right almost obsolete.
If \left and \right were automatic, then whenever you needed something less-but-often-still common, like [0,1) open intervals, or physics <bra|ket> notation, that would require kludges.
Whether or not it is a smart decision is a different argument.
I use templates so my editor shims in the above anyway, at the cost of lots of noise.