Actually, pentode/triode isn't referring to disabling some of the power tubes. Most power tubes run as pentodes, which are just improved triodes. This improvement in operation allows pentodes to get a much higher amplification factor than its predecessors. The screen grid is one of the electrodes that make the pentode so much more efficient than a triode. By connecting the screen grid to the plate, we can force our power tubes to run as triodes. Once disabled, the pentode suddenly doesn't have as much gain, hence a drop in perceived volume.
Thus, it shouldn't wear the tubes out faster, and all tubes are still working. But it will sound different. As triodes aren't very efficient at amplification, they don't amplify the high frequencies as much. That's the whole purpose of the screen grid; to improve efficiency and thus high frequency amplification. This causes a sort of muffled sound. FWIW, YMMV, etc.
