Yes, it is. It is done all the time. Make no mistake about it. Aluminum on the atomic level (or a bit more if a cook is overzealous with utensils in the vessel) gets into the food cooked in uncoated...
Yes, some cooks prefer them for their lightweight design.
Addition:
Yes, as long as it is not cast aluminum.
The cast aluminums are high in oxides and are also porous. Very seldom are pots cast...
Cast Aluminum is made by pouring heated molten aluminum into a mold. During this process, microscopic air pockets form in the metal. This means that the resulting cookware items will hold their heat...