To be honest, I never understood why Hitmonchan was like #1 in usage some time ago and I felt like he was overrated. But seriously, E Rank ?Im just going to say that hitmonchan actually doesnt have any niche over kabutops, cryogonal, or even sandslash, i was actually thinking of putting hitmonchan in E rank thanks to how bad it is, because ive never seen anyone use hitmonchan to any success since mid BW1.
You can say that Kabutops is a better spinner than Hitmonchan and that Hitmonchan is awful in Cresselia's meta, and really, you'll be right on both accounts. But in my opinion, you can't say that Hitmonchan doesn't have a niche. Due to their differences in typing, bulk and offenses, Kabutops and Hitmonchan force out different foes , can switch into different things, have different checks and so on. Just because Kabutops is better overall, doesn't mean Hitmonchan can suit your needs in specific situations. For example you might want a spinner that can switch into Steelix or Aggron, or you might be using a slow Scarfer like Emboar and appreciate Hitmonchan's ability to revenge LO Omastar.
RU really has few spinners available and having Rapid Spin with decent to mediocre utility should be enough to avoid E-Ranking.