I don't think you have to go for purely functional.
Jane Street, for example, went with OCaml in part because the business people found the code easier to review than other languages.
Jane Street, for example, went with OCaml in part because the business people found the code easier to review than other languages.
That said, the fact that OCaml is used by Bloomberg and Facebook (on multiple apps and products) alone make it not niche http://www.ocaml.org/learn/companies.html
That plus ReasonML's growing popularity, which is essentially just a syntax extension of OCaml, makes it even further not niche.
And that counts in all sorts of ways. You'll have much more trouble finding answers to any questions you have. You'll have less chances for community meetings or educational content. You'll definitely have more trouble finding new developers for your projects, both in business and in open source.
Erlang doesn't support mutation at all, for example, and uses immutable types for everything. But it does let you send messages to other processes, which is a side-effect, so Erlang is not purely functional. But that doesn't mean that JavaScript is somehow just as functional as Erlang. Because it's not.