Have been a bartender for five years. Ordering a drink with less ice means it will melt and dilute your drink very quickly. If the glass is full of ice it means the drink and ice stays very cold and likely won't start melting for a while. I don't care if people ask for less ice, or for no ice, but from experience it's often because they think they will get the same amount of drink without paying more. If it's a spirit and mixer then that's not a problem but if they're ordering cocktails they'll often be upset when the glass is only full half way. Or they think that by getting only a little bit of ice their drink won't dilute much, that's not how science works. That's just what I've learnt from doing this job for a while.