In terms of economic incentives toward SC bleeding value out of MC, which appears to be a major misgiving with SCs; it has already happened with alts less so now (but I think it will accelerate again when a bull phase returns) and also with counterparty, mastercoin and bitshares that are in essence one-way pegged assets.

So we already have it in the protocol to create 1-way pegs, you can't stop that and they have shown how they siphon value out of the main chain, e.g. mastercoin has been many multiples of its btc 'cost' at times. In all probability, making a 2-way peg facility available that makes it more efficient to go in and out of the MC makes it less likely than it currently is for value in total to exit the MC via super successful 1-way pegged assets. Therefore in that analysis, as the protocol currently exists it is more vulnerable than one allowing for 2-way pegs. You could consider it as the 2-way peg closing the economic value loophole for an attack with very successful 1-way pegged assets ... shhhhh. After a successful soft-fork allowing for 2-way pegged assets any entity proposing 1-way pegged assets will be viewed skeptically and economically disadvantaged by the market perception of losing interoperability with the MC. The market will prefer 2-way pegged assets over 1-way pegged every time, and that is a net benefit to the total value proposition of bitcoin surely.