I didn't see anyone mention this here, though I might be wrong.
In tWoK prologue Brandon explains(through Szeth) why humans can't hold srotmlight forever, and hints to why the Fused seem to never run out of voidlight:
"Stormlight could be held for only a short time, a few minutes at most. It leaked away, the human body too porous a container. He heard that the Voidbringer could hold it in perfectly."
I don't think that the Fused don't waste voidlight by using their surges, and I don't think that they're being constantly invested all the time. I believe that they're being invested periodically, though I have no idea what is the source(like spheres and the highstorm). And that they seem to always be invested because they're so much more efficient with the investiture.
If I'm correct, Venli is going to be a powerhouse next book.