Vaal gives you the chance of granting you 2 additional map properties, chance to have their properties randomized or become unID'd (30% more quant).
Most people chisel, scour and alch til they get what they want, then slam vaals. As long as you're running sextants/mapmods/etc, even if you're spending 30-40 chaos per map average, you'll still profit. At least I did when I played 5-10x 100s per league.