Maybe they should scale from level and use, allowing different people to become more potent with the skill they use. Those who focus single nodes gaining damage while those who want to use pulse instead of single target gaining that. Also it seems that using purifiers limits your xp gain compared to pulse, so its better for personal progress to focus on using pulse repeatedly instead of actually progressing the wave. Purifier seems like an inhibitor so you only get so much xp from the investment.