It's simple: either you care about validation, and you must validate everything, or you don't, and you don't validate anything. Sidechains do not offer you a useful compromise here, as well as adding huge delays and conplexity.
As noted to Adam last night - although I agree it adds complexity - side chains are one solution that will indeed help with scaling long term. Similar to the graph you see with git repos and merges, having aggregation chains that arbitrarily fork and then rejoin the main chain are both feasible and useful.
That code & future is a ways away from production, so doesn't help us here. Still, let's not dismiss it as a solution either.
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Jeff Garzik Bitcoin core developer and open source evangelist BitPay, Inc. https://bitpay.com/