Well, my proposal wasn't to mix 2 transactions, but maybe 10, or even 50. And then once you add the change addresses (at least 1 per wallet), it is no longer so easy to figure out what was used for what, and what belongs to what.
Couldn't you say the same thing about this mixing? It could be expanded pretty easily to have Alice's mix offer be "Hey, I'm running a 5 BTC mixing party. Let's get everyone in on this same transaction." If a lot of people are throwing in their 2's and 3's, it'll get difficult to find the original pairs.
Not really, because once I do my 100btc transaction, I have to combine all those in my own wallet. So mine are again identifiable as mine. The "bad" operation is to actual combine. So my idea was, make the combine operation more anonymously.