The decoder might not be running strictly all the time, but I would wager that for some applications at least it doesn't make much of a difference. For HPC or DSP or whatever where you spend a lot of time in relatively dense loops the uop cache is probably big enough to ease the strain on the decoder, but for sparser code (Compilers come to mind, lots of function calls and memory bound work) I wouldn't be surprised if it didn't make as much difference.

I have vTune installed so I guess I could investigate this if I dig out the right PMCs