DNS and Your IT Structure

I use 1and1 for hosting and have a shared hosting package, which I absolutely love. I don't get enough traffic on my sites or do enough different things that I need a dedicated box. I also happen to do my domain registation through 1and1. Recently, however, I've run into a situation where a client wanted to use 1and1 as their host but absolutely refuses to change their DNS to comply with 1and1's nameservers. This is something I never would have thought would be a problem, before it actually became one.

What do you other developers think? Is it "best practices" to keep DNS separate from web host? Is there any real benefit to this?