That is not particularly rational. It makes moving hosting providers harder and many provide very different dns services. You might want an api or support for different record types that otherwise good hosting services dont provide. I use zerigo just for dns because they are really goid at it...
Managing DNS completely separately is fine, too. I just don't think it's a particularly good idea to use your registrar as your DNS provider unless you're also using it for hosting. I'd much rather point my registrar at my host's DNS than to have to update DNS records every time something changes on the host.
