Who said you cannot restore the firmware? If you use a boot image such as ClockWorkMod, it gives an option to backup your existing firmware that you can restore from later.
Also battery backup gets reduced because of low quality ROMs which use improper battery caliberation or use apps that are known to drain battery. You should use reputable ROMs such as Cyanogenmod which are more well crafted for your hardware. Stock ROMs are even better than bulky bloatridden ROMs.
Also, updating the ROM allows you to install ROMs with the latest android version and is the only way you can get a newer version of android if your manufacturer doesn't make it available for your model.