It's not that the grammar is ambiguous, it's that the thing you're parsing may not even be in the language generated by the grammar. But you want to parse it as well as possible, and there may be more than one way to do that.
wow, absolutely not. if i'm parsing a programming language I want to basically stop and bail from the current context as soon as possible. there is basically nothing to gain by continuing.
if the shit i'm parsing isn't in the language there is no reason to continue. its invalid.
If you are writing a code editor you might want to continue parsing it. Or maybe you detected an error, but you want to report more than one error at a time for a better programmer experience. There are lots of use cases for partially parsing a computer program.
I had already explained why one might want to do this:
"Consider the problem of parsing a C program that has NOT been put through the preprocessor. You want to be able to do this for structure editors and source-to-source transformation systems."
One cannot in general parse un-cpped C/C++. But that doesn't mean it's useless to do as well as one can, even if it cannot be parsed exactly. Parsing is for more than just compilers.
