I would like to propose a case AGAINST the idea of shared Namecoin/Bitcoin mining.

The main reason I think Namecoin and Bitcoin should be kept apart, is perhaps to preserve the potential future value of Namecoin.  It has little to do with Bitcoin.

Merging the mining of Namecoin and Bitcoin is a bit of a marriage.  Namecoin perhaps has a better marriage candidate: Tor.

Why Tor?

Well, for one, the idea of a peer-to-peer DNS registry isn't something the world is asking for.  While recent domain name seizures have spooked many of us, they haven't really created any serious practical problems that need solving.  Very few websites are candidates to get their domain names seized, and those that are can protect themselves simply by registering themselves in other top level domains.  (This is something that they'd have to do under the Namecoin plan anyway).

On the other hand, Tor lacks a working DNS system, and forces people to use cryptic URLs to access its hidden services.  Just look at the Silk Road - their address is nonsense characters, and so they suffer from attacks like people hijacking links to them and sending people to phishing lookalikes, in the hopes that random characters XXX will look close enough to YYY that few will notice the difference.  On the other hand, if Tor had a cryptographically sound DNS system, it could offer addresses like "silkroad.onion".

The Tor project itself says about domain names: that you can pick any two of the following: decentralized, secure, memorable.  And goes on to explain why enjoying all three is likely impossible, referring to "Zooko's conjecture".  Namecoin could just give the Tor project all three.  Tor project even invites it to happen: at the same link, it says: "Perhaps one day somebody will implement a Petname design for hidden service names?"

As a separate project, Namecoin has a fighting chance of actually being proposed, adopted, and integrated into the Tor infrastructure, as well as any other anonymizing project.  If it were married to the Bitcoin block chain, I just don't see how such an integration would be as attractive.

tl;dr - The world isn't demanding a .bit domain just to get access to Wikileaks when Google gets them there just fine.  Tor could really use a good DNS system, but marrying it to the Bitcoin blockchain will serve to make it unsuitable.