Sure there are situations where you might not use the CGI module, but
Optimizing parameter parsing is not going to give you any significant speedup unless you've got tens of thousands of parameters and hardly do anything with them.
URL decoders are not hard to code, except that people seem to forget to implement the full spec. (See for example & vs ; separators)
Lack of experience is not a valid reason to keep code that could be replaced with a well-tested core module.