[saurik]

Normally, developers are able to get away with monkey-patching the compiled code in memory; but, in some cases, you simply need drastically different settings. A while back (important, as Apple now added this feature... at least I think) some developers managed to sort-of-almost recompile WebCore (with some horribly reverse engineered hack for the redacted parts) in order to add RTL (right-to-left text rendering) support: their goal was to get Hebrew better supported (a goal that was mostly successful, and was used by a large number of people in Israel, with nearly everyone in Israel using it).

Even when you take the monkey-patching route I generally prefer (as it allows you to more easily stack patches from multiple developers/vendors), modifications to a C++ library like WebCore are much much harder if you don't have the original source code. In my case, I add a new <script language=""> handler for "text/cycript", a dialect of JavaScript that allows inline Objective-C syntax (similar to Objective-J) that bridges to native code, so it can be used by people developing HTML desktop widgets.