>> FreeBSD, I do understand your message
No, you don't. Your question was all about gluelessness.
>> so zoneedit.com points to my dns server?
zoneedit.com doesn't do delegation. In this case, zoneedit.com is equivalent to myisp.com in my previous example. As djb mentioned, gluelessness is extremely bad and possibly 70% of visitors won't be able to reach those 50 sites of your clients.
When running a inconsistent and unreliable DNS servers like that, why not releasing all 50 domains and tell zoneedit.com to host them? Apparently, it's better off to get static IPs and run your very own DNS servers. Don't tell me you can't get static IPs. There is no free lunch in this world, if you wanna play this toy, you need to pay for it.
>> I only need to take care of the a single ip change with zoneedit.com
I know what you mean. Instead of editing 50 + 1 domains thru zoneedit.com, it's much easier to edit just 1 domain (yours). But gluelessness is very bad, just don't do it. Further, with dynamic IPs, your infranstructure just doesn't allow you to run your own DNS servers.
Now please read thru this once again -> http://cr.yp.to/djbdns/notes.html