Hi Pierre
Certainly for public performance you will need a full PA system however for home use the inbuilt mic is fine. As for damaging the speakers, well, I doubt very much that more stress could be put on the speakers by your voice than by, for example, the rock drawbars.
By the way you can add additional top end to the mic output by adjusting the overall equalizer but of course this affects the whole keyboard. I have done this and don't mind the resulting brighter sound.
Have you also noticed that using the multi effect you can do all sorts of things to your voice.....add chorus, modulation, etc etc, but only one at a time.