[alexti]

In most cases, you know what the spells you request cost and how many gems to give. In a few cases, when that's not true, you need to spend half a day thinking what you're going to cast, so extra minute to calculate required gems won't make much difference. And anyway, you're probably going to give your mage few extra gems to handle unexpected. You don't want to lose a major battle just because your main spellcaster got hit by something fatigue increasing.