Because for those who didn't know better Müller appeared well-informed and it was convenient to cite him if he was on your side or you were writing a mainstream article and too lazy to find another source, especially after Groklaw shut down.

At least the Oracle vs. Google case revealed that he was actually a paid consultant for Oracle and Microsoft. It might be bad form to attack his character rather than his arguments, but it does save time when dealing with Müller.