Hennet realizes that he has no way of escaping from the ogres. The ogre-mage had used its wand to dispel his fly spell, and Hennet's other spells had run out. Except for limited wish.
Actually, Hennet can use any of his lower level spells on his 7th level spell slot. So if he has DD or Teleport or any such thing on his list, he can use it.
Still, Limited Wish is a pretty good spell. It's not something you will use often (a negative point for the sorcerer) but by the time you get it it can just sit on the back of your list, to be taken out triumphantly if you are ever taking a heavy beating. 300 XP is not that much at higher level, so you can pay the price every now and then. The biggest prize of this spell, IMO, is its ability to reproduce ANY spell of 4th level or lower. This means you can reproduce a divine spell or something else not on the Wiz/Sor spell list, ie
cure critical wounds (a life-saver),
spell immunity,
modify memory,
reincarnate,
speak with dead,
restoration, etc. I admit, I haven't used it in practice, but if you play a hard campaign, you are likely to use this stuff at least once. If it saves your life that one time, it's worth it in my book.
Acid Fog is also a good spell, but personally I see better at 6th level. True Seeing, Chain Lightning (if you're an evocation-type), Eyebite, Shades, Disintegrate, Greater Dispelling all beat Acid Fog to the spot in my book... It all depends on your spell list, of course. Acid Fog is the sort of thing my sorcerer tries to get scrolls of. It has no save, so being in a scroll doesn't really matter much.