I mean, for me it helps that Kuwabara has some legitimately cool moments, so while he definitely gets worfed a lot, it feels less like he's absolutely worthless, and more so that he's clearly the weakest protagonist but still keeps pace to at least some extent. It also helps that Yu Yu Hakusho has lower power scaling so it doesn't seem like things go absurdly beyond him too fast. Plus he also does serve the "Main Characters best friend" role so he has some relevance beyond combat effectiveness.
http://tvtropes.org/pmwiki/lb_i.php?lb_id=13239183440B34964700 Alfric's Fire Emblem Liveblog Encyclopedia!YYH's power scaling is still pretty absurd - there are massive differences between classes (about 20 times from just from A to S, and the difference from lower S to upper S is far greater than E to S), A class are supposedly able to devastate the entire planet and have power equivalent to various gods from religion and mythology, and the power levels given in the manga imply that the difference between Yomi and his right hand man is nearly one thousands times. The same amount that Freeza had over Captain Ginyu!
It's just that power-ups are extremely easy to get, and get easier further into the series. Seeing Yusuke die made Kuwabara, Hiei and Kurama rocket up to middle A class from being C - B class characters. Then with some months of training they can make a bunch C class characters into middle S class fighters, and in the manga Kuwabara reaches S class off-screen while studying for university.
Here
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They made Broly canon without actually making Broly canon and added female super-saiyans at the same time
edited 21st Jan '17 7:40:54 PM by Hylarn

Well, there's the invisible man, which had interference from both sides so it's kind of fair.