So as I understand it: I have 1BTC. The blockchian forks, I now have 1BTC-core + 1BTC-XT. I continue to do nothing, and this balance remains. After a time, the value of these different coins (in fiat terms) diverges. Someday, I decide to sell for fiat. If I've waited long enough, I think one of those coins is probably worthless, so I probably don't want to bother separating them. If I want to separate them, I can do so by the method in the OP.
There is no a way for the market to instantly decide which coin is worthless, this can be done only via trading. If you have balls of steel and can wait long enough then you indeed don't need to bother separating your bitcoins.