I think the case of a developer knowingly reinventing the wheel "because he prefers his own method" is pretty rare. Mostly it's done out of ignorance, and sometimes out of stubbornness.

Is it all that bad? Yes. Why? Because the existing wheel has most likely been crafted over time and has already been tested in lots of circumstances and against lots of different kinds of data. The developer(s) of the existing wheel have already encountered the edge-cases and difficulties that the reinventor can't even imagine yet.