Are you mainly saying that the incentive structures for mining change in unpredictable ways if speculative assets are built straight into the money itself, rather than those assets simply being denominated in the money, like how the DOW is denominated in dollars? If that's what you're saying, I might agree. I'm not sure I see any problem with the incentives, but I also can't rule out a problem. The precautionary principle. This would extend to things like Counterparty, yes?

Wait, I'm pretty sure you said before that it's fine for things to be built on top of Bitcoin. So I'm confused about whether your argument here is just an extension of your "Bitcoin will only ever be used as money" thesis or whether it's specific to sidechains due to merge mining or for some other reason.