Jane Street uses Ocaml professionally in a financial trading application. Here's an excellent write-up on why they chose it, its advantages, examples in action, and a few limitations.
Esterel also used it to develop a highly-assured code generator for DO-178B-certified software. That's one of the toughest projects you can do and it made it easier.
Hope this helps to show its practical value. I've always thought Ocaml's design had some of the best compromises between how we'd like it in theory vs getting things done.
https://queue.acm.org/detail.cfm?id=2038036
Esper Tech is another one using it and wrote up why here:
http://tech.esper.com/2014/07/15/why-we-use-ocaml/
Esterel also used it to develop a highly-assured code generator for DO-178B-certified software. That's one of the toughest projects you can do and it made it easier.
http://www.researchgate.net/publication/220802806_Certified_...
Hope this helps to show its practical value. I've always thought Ocaml's design had some of the best compromises between how we'd like it in theory vs getting things done.