HackerNews may not be the best place to sample for UML usage. It consists mainly of two communities that have a bias against formal engineering methodologies in the development of software.

I won't comment on the pros and cons of UML. Instead I'll invite you to ask yourself a couple of questions.

1. What other clients do you support who have similar characteristics as this client (and may therefore also benefit from UML support)? If the number is significant in terms of impact to your bottom line versus the time you'd have to spend implementing it, then you should consider it worth your time, and view it as an opportunity to up-sell (if you can) or keep existing customers.

2. Do you intend to attempt to move into supporting large enterprise, and especially government contractors? If so, you might consider UML support just because it is ubiquitous there.