btw (I know you were predicting JustusRanvier) but I dont think one can characterise the op_spv is to the disadvantage of blockstream competitors.  Its an op code, anyone can use it to make sidechains or other things. 

Other things: you can also use the op_spv opcode to enhance security and efficiency for SPV clients like smartphone wallets completely separately from sidechains.  And to fast catchup headers also, the compact SPV proof is work-preserving.

If for example OpenTransactions sees a use for it, or ethereum wants to switch out ethers for bitcoin (i imagine the developers are attached to their premine for now) but perhaps Mastercoin or something thats market cap is falling, seeing less use and the ICO people probably mostly dumped by now (if thats a fair characterisation, dont follow it closely) maybe they might want to migrate to a sidechain, thats all expected and good.  Permisionless innovation is the point of sidechains.  You could even one-way peg the remaining mastercoins into a sidechain to allow their residual tradability.

If you mean it allows bitcoin to more easily incorporate features, well most alts dont really have features, or not meaningful or useful/non-broken ones, but there are some feature coins and bitcoin 2.0ish coins that do.  I dont see how extending bitcoin is unfair - they're all leeching off and copying bitcoin, which was the actual innovation - why cant bitcoin take little bits of innovation back too?

Also alt-coins or feature coins can have sidechains off them also for beta versions, or secondary functionality if they were actively developed.

Adam