ArticMine, the merged mining for Namecoin with very minimal validation can't be compared to the extensive CPU resources required to verify smart contracts.
I have no confidence whatsoever in Rootstock being merge-mined by Bitcoin miners. Fuhgeddaboudit.
Rube Goldberg machines suck.
Time will tell, and I am also unsure if they will get the hashrate. They will need a percentage of the hashrate comparable to that of Namecoin in order to be secure. otherwise they could get attacked for the reasons you cited above. Of course if it fails or even if it works then one can learn from it and build something better.
Edit: For starters they also need a coin that can scale as the main chain for this to work and Bitcoin does not meet this requirement.