I have had by chance lighter racquets in my bag (simply different racquets that weren't my main racquets at the time), but never for the acute purpose of having something available that was lighter should my heavier stick become too cumbersome.
Having said that, I think I actually could switch to a lighter frame during play if I felt I would benefit, as long as it's a lighter frame I liked and wasn't drastically lighter. After years of being a spec junkie and obsessing about little details, I've realized I can play good tennis with a fairly wide variety of frames. Of course I have preferences, but overally I can adjust if need be, or simply just to try new stuff out or revisit old friends.
Also, as someone up thread stated, sometimes it actually helps to have a heavier, more substantial racquet as relief rather than a lighter one, if things are somewhat out of control. Offers more stability, enables you to simplify things, shorten strokes.