Here are a few comments from the 3.5 FAQ on the subject(s) being discussed here:
Activating an energy power requires a standard action, but once you activate energy power, the power works until you use another action to deactivate it. You can activate or deactivate one of these powers on up to 50 pieces of ammunition at the
same time, provided that all the ammunition is in your possession, all the ammunition is the same kind, and all the ammunition has the same power.
A burst weapon’s burst power is use activated and it works even when the weapon’s energy power is not activated (see the last sentence in each power’s description).
The energy from a flaming, frost, shock, flaming burst, icy burst, or shocking burst weapon never harms you while you’re wielding or carrying the activated weapon (see the power descriptions), and it will not harm your equipment. If you lose or set down an activated weapon, the energy it produces will harm other objects it touches, so it is best to deactivate it first.
There’s nothing illogical about a flaming, frost, shock weapon (at least not within any framework that allows weapons to generate energy in the first place), and there’s no rule against such weapons (think of the weapon as having fiery, frosty, shocking flames). The character creating such a weapon decides how it can be activated. Most such weapons probably are made so that the wielders can activate all three powers
simultaneously, or activate them one at a time, as desired.