True, but any # of reasons for Obi-Wan leaving could've been concocted as well, and made sense... He could've gone to Utapau after the Separatist leaders even, no Grievous necessary, had it been known they were there, and they simply escaped to Mustafar... That would tie in with why Palpatine wanted Anakin appointed to that job as well...
I like Grievous myself... I just think he wasn't a necessity, but he worked well for what he was. He was cooler in the cartoons than in the film though, as far as "wow" factor would go.