One reason: It's fun.

Many reasons: Solstice cannot be directly attacked by anyone except Mythslayer, and if he does move, he'll get killed by Jim or 21HP Ice Hydra, because we won't really move Mythslayer until after he has used Slay.

Preserving either Grandma or Fooruman would allow Hellfishy/Foshzzle to be cast on Solstice (or another Ice Hydra). Our aim is to win by killing everybody, isn't it? So why not Ice Hydras because they have the lowest HP, and don't you just love the sense of fulfilment when you know you have wasted as little spells as possible before the end of the game?