Adam,

1.  what's your view on the maximum #SC's that can reasonably be MM'd by the Bitcoin mining network?  i'm not aware of any capable mining software in existence today that would allow multiple SC MM'ing besides the simple Namecoin experiment, which is a public service altcoin afaic.

2.  given that individual miners have limited resources and technical skill, won't MM'ing force solo miners into pools where these skills and resources will be more centralized?

3.  we've already seen a 10x drop in the prices of mining hardware as a result of the stiff competition and profit crunch.  this is a natural cyclic effect of the game theory which drives competition for BTC rewards and tx fees.  you say that SC's can help these profits by helping to extend demand (possibly).  but the fact is, you're compensating for what is a natural result of the incentive structure as the rules are currently written.  you're changing the rules of the game, midstream, that changes the market assumptions behind mining.  i actually consider these extremely thin marginal profits to be in an exquisite balance today.  considering only the positives of this change while ignoring the possible negatives is naive as this is an extremely complex topic.

4.  51% attacks will be much more profitable and tempting with SC's, imo, as they don't risk destroying mining pools main source of income, the MC.  short selling the to be attacked SC on an exchange while stealing its scBTC at the same time could be quite a profitable scheme. 

5.  if a SC like Zerocoin/Zerocash gets declared illegal by the US gvt, legit pools that can't/won't participate in an illegal SC might have an actual incentive to attack it to destroy those users and their scBTC that are in fact using Zerocoin/Zerocash illegally.  given that no SC will ever have 100% MM, 51% attacks become much easier to execute.