WHat do you mean by reasonml for server apps? If you mean running js(output of bucklescript) then no it will likely not benefit node users.
For instance this fast OCaml http server: https://github.com/inhabitedtype/httpaf
And you can even share types between client and server :)
They lost the Flow-TypeScript battle. And they learned, they wanted even more typesafety I guess.
Since TypeScript also compiles to JS, bringing "something a little less like JS, but a lot more safe" to the table seemed a better idea that pushing "unfamiliar syntax, but heay, it compiles to JS"
If there is any battle, I predict will probably be between Flow and Reason inside the Facebook org because of competing solutions.
There's even a browser extension which converts OCaml syntax to ReasonML and vice versa in docs.