It just seems like having this debate is about repeating the same points over and over again. Case in point:
Indeed.
Assuming they approximate the design goal, they offer all the advantages of both Bitcoin and alt coins.
As I mentioned to davout:
Bitcoin miners can attack sidechains at very low cost to themselves, as they can earn BTC mining Bitcoin main chain WHILE doing the attack on the sidechain. Furthermore, a successful >50% attack on a sidechain is much more destructive than one on Bitcoin main, allowing the attacker to steal all of the BTC backing the sidechain. A sidechain is also likely to not have 100% of the network hashrate of Bitcoin merge-mining it, since it requires extra effort for a Bitcoin miner to setup to merge-mine another chain, so it will be easier to >50% attack.
For all of these reasons, sidechains are not as secure Bitcoin main. I recommend you read the sidechain whitepaper published by Blockstream to see the risks that sidechains face.
I see the need for merge-mining on a sidechain as only necessary to interact with and support it's backing store (Bitcoin.) That is, what I would call their 'backing store layer'.
Bitcoin mines because POW is it's only backing store. Fiat has the power of government for backing. Gold has {n} Avogadro's numbers of difficult to synthesize atoms. Bitcoin has sha256 hashing.
Sidecoins have Bitcoin as backing. Thus, they do not need to use POW (sha256 or otherwise) to handle the internal accounting, and all manners of fairly secure and verifiable options exist here. I'll expect to see many such options explored. People who think that POW mining on a blockchain is the only way to achieve this, I believe, only know a hammer and thus see every problem as a nail.
You also clearly didn't read my statement on where I do and do not need 'military grade' security. I keep some excess fiat in PayPal, but not my lifes' savings. If they fold and/or steal my money I'll be pissed and I'll try to get it back, but it won't hurt me much because I balance my risk. Indeed, I do the same for fiat in banks. That will be very much the case for sidechains. I'll mainly choose to use sidechains based on my desire to support a particular effort which sponsors them. I'll not put much money into any particular sidechain for some time, if ever, so I simply don't need high level security. I'll use native Bitcoin for my deep storage of real value...unless the bloatchain guys win the war that is.