You can disagree, but it's because you don't know what you're talking about. Sorry. Trundle's clear time is extremely good if you're timing your Rabid Bite properly (immediately after an auto-attack). He clears the entire jungle with near-full health and he's a nightmare to 1v1 if you're playing a bruiser. His rabid bite steals attack damage and his ult steals health and resist.
He counterjungles by simply walking into the enemy jungle and beating the **** out of the enemy jungler. Low health? He'll just walk off into his own jungle only to appear full health again 1 minute later.
And about the high single target damage, it might not be the best for clearing own jungle, (with blue and constant Q's still pretty fast) but it's THE BEST for stealing the enemy jungle, where you just want that important one down fast, not to mention it enables him to do the dragon extremely early. He just needs the wriggles and then it's byebye dragon.