A seemingly elegant design can quickly begin to bloat with unexpected content or break under the weight of actual activity. Fake data can ensure a nice looking layout but it doesn’t reflect what a living, breathing application must endure. Real data does.

If the copy becomes distracting in the design then you are doing something wrong or they are discussing copy changes. It might be a bit annoying but you could tell them that that discussion would be best suited for another time. At worst the discussion is at least working towards the final goal of your site where questions about lorem ipsum don’t.