I think Horwill has been a very good captain at Super level, interestingly when he was playing better personally, and probably had a far clearer idea of what was wanted from the coach, not to mention closer ties with his team-mates. I think his "death or glory" style of leader works really well there.If you are looking for reasons to bag him yes. You could also say that he has been at the Reds for years doing a great job there winning a title and maintaining a spot in the top for consecutive years despite missing a key playmaker for half of each season.
At Test level he has not been as dominant a captain, probably for many reasons, not least of all his own injuries, the coach, the disjointed goings-on in the squad and so on. He doesn't seem to enjoy as clean a relationship with refs in Tests.
If he gets back to his best form, and the Wallabies start to get some confidence and momentum, I'm sure he could stand out as leader again.
For now, I completely agree with McKenzie's decisions to bench him (pre-Simmo injury) and keep Mowen as captain. I'm sure we'll see better from him soon.