Re: This might be a stupid comment, but...
Naaaaah! I know you were being rhetorical, but I'll answer anyway :-)
I do personally use a registered domain for my internal stuff, but it's not necessary.
One of the reasons is so that I can keep the internal / external DNS configs on one machine. However, the domain is slaved to internal nameservers, and all internal machines use internal nameservers so that the internet is not a dependency.
In fact, whilst I agree about the domain mess, anyone who's internal systems could break due to this (or to the other often mentioned thing about some ISP's returning a default IP instead of NXDOMAIN on unknown addresses) really needs to fix their setup anyway..... If your internal printer fails because network solutions (or whoever) put a * wildcard under .com, then you have other potential security and reliability issues to fix!
As for the tongue-in-cheek comment about having internal mailservers listen on all domains, I don't see how that is relevent to the situation you are commenting on... Surely, more of a hypothetical question is should your external mailserver accept stuff routed to names assigned to registered internal hosts? !!