Sidechains are altcoins, which use Bitcoins as their basis. By subscribing to the sidechain, you subscribe to the same ethos as Bitcoin... namely, the vast number of coins owned by Satoshi and now the core developers (some of whom were rumoured or seen to have made large purchases of Bitcoins with the formation of Blockstream).

I can't wait for the multitude of sidechain forks that all compete with each other. Sidechains themselves require incentives to operate securely, such as fees and the continued security of the Bitcoin network, so I'd guess we'll start to see a lot of the same issues in sidechain space as we do in altchain space. I really don't think there's any difference between the two. You can also issue your own assets already on Bitcoin (e.g. coinprism, counterparty), so the notion of token issuance for use in general scamming will surely appear on sidechains as well. You can't out-technology human speculation in finance.

The other issue with sidechains is that effectively everything can become a sidechain of anything else if you push foreign coins onto Bitcoin mainnet as coloured tokens. I've been calling this "bilateral two-way peg" for a while, and I'm not sure that there's any way Bitcoin mainnet could prohibit it after they add sidechain-related OP codes. The real (negative or positive) value of sidechains remains to be seen, and I think we haven't even touched the scale of redundancy that might occur in the future as a result of them.

The good thing is that the Blockstream guys are doing some pretty neat cryptographic research, in any case.