How big of a deal is blocking copy-pasting on a website?

I'm working on a website on which the client wants to disable copy-paste.

It's not to do with security or passwords, but that the data on display is valuable if taken en-mass, though the user can only access the items one at a time. (i.e. There is no page a user can access with multiple items visible at the same time)

I have advised that its probably bad UX, and that it wont block screenscrapers or similar, but at the end of the day it's his decision.

How much of an impact does blocking this functionality have on a website? Is it likely to turn off users from engaging, or is it a non-entity and I am putting too much value in this?

If its a big deal, any recommendations on how to advise the client?