I'm not sure it is right to speak of "most people". Many people use OCaml these days. For instance, I have the impression that in work on program verification and static analysis OCaml is more popular than Haskell. Coq is written in OCaml, for example. The Haskell community much more vocal though.
Harper has strong opinions, but he has a few points. Haskell may be great, but one should be aware that it is not an improvement on SML in each and every respect. For example, SML has a much better module system than Haskell.
Perhaps OCaml gets a lot of use in that community because a lot of that community is French -- that was my experience, anyway. And a lot of the folks in that community don't seem as vocal (to this side of computing) only because they are using different channels and complaining about different stuff!
It was actually surprising (and somewhat annoying) that Twelf is written in SML.
Harper has strong opinions, but he has a few points. Haskell may be great, but one should be aware that it is not an improvement on SML in each and every respect. For example, SML has a much better module system than Haskell.