I think it is obvious that the consensus here is that the tech was giving you incorrect information, for whatever reason (I will not speculate, nor call him an idiot - he may have been following orders, for all we know).
If you still don't believe us, please feel free to continue to do as the tech says - and keep wondering how your phone works great for a couple of seconds, minutes, or hours, but eventually goes back to being crap - while mine, overclocked at 1200 MHz with a performance setting in SetCPU and with 170+ apps installed manages to give me 9-10 hours a day on moderate use throughout the day on my battery....
Choice is yours.