I have an example of how noweb actually made it possible to code something that would have been very hard to maintain otherwise. In numerical applications, one typically has many nested loops, the inner statements of which are executed tens of millions of times. It is imperative that one avoid branches inside those loops, since fast execution is the top priority in these codes. Consider this example: 
<conditional inside loop>=
...loop control stuff...
if (x) {
  <Do this>
}
else {
  <Do that>
}
...more loop control stuff...

To avoid the if, without noweb, I would make two copies of the loop control code, one with <Do this> inside, the other with <Do that> inside, and with the if statement outside of the loop. But that means maintaining two copies of identical loop control code, which is unacceptable. So instead I do this: 

<conditional outside loop>=
if (x) {
  <Do this loop>
}
else {
  <Do that loop>
}


<Do this loop>= (<-U)
#define WANT_TO_DO_THIS
<Generic loop>
#undefine WANT_TO_DO_THIS


<Do that loop>= (<-U)
#define WANT_TO_DO_THAT
<Generic loop>
#undefine WANT_TO_DO_THAT


<Generic loop>= (<-U <-U)
...loop control stuff...
#ifdef WANT_TO_DO_THIS
<Do this>
#endif
#ifdef WANT_TO_DO_THAT
<Do that>
#endif
...more loop control stuff...

So I end up in the tangled source with two copies of the loop control code, just like I would have to do by hand without noweb, but I only have to maintain one copy. Couldn't do it without LitProg.