That's only partially correct. As stated above you get your runes no matter what spell your using, and you can sell the runes since you're not using them to pay for the airs, as well as have more profit than you would have had using slayer dart. Also, slayer dart hits (Magic Level)/10, truncated. That means you can hit from 15 to 20, depending on your magic level. (If you're 99 magic and use potion you can hit 20s.) For someone with low magic this means you hit just as high with firebolt and chaos gauntlets. But with wielding chaos gauntlets you lose the opportunity to wield barrows gloves or w/e RSD glove you may have.