I think the rate of fire should be increased for the incendiary. Then people wouldn't save it to the very last and only upgrade it when everything else has been upgraded. Most, if not all, of the other defense towers have something about them that makes them attractive to upgrade. Even the Tesla. Although it's health isn't great, they can target multiple enemies. So in the same respect, I think if incendiaries had something special about them, like a higher rate of fire, they would be more attractive to upgrade earlier.