Or just turn them off?
I think we're potentially coming down to the functional difference of bailout vs. deco gas. I don't like your approach because it assumes that I'll be gas switching deliberately; which is often the case on open circuit. On closed circuit, I could be wedged into a tight spot on a wreck (in fact being wedged and compromising my loop is one likely scenario where I'll want to be able to get onto my bottom gas). If I can get the second stage in my mouth, and the tank is charged, I can get gas flowing because the inline shutdown is right there. I may not be able to get to the valve as quickly.
Your approach makes good sense for decompression switches, less for bailout scenarios.