The sleep spell, is actually for a rather pointless side quest. To get a treasure chest that serves no purpose in the game. It's not really important to the story. It's my least favorite 'puzzle' in the game, since it requires listening to rats for a random message, and if you take too long, and they don't give you the message, the ship might sell off before you can leave. It can take up to a half-hour to get them to give their message about the treasure. If you cast sleep spell, the rats don't show up. All that wasted time for an optional item that serves no story purpose. If you never hear the rats, and are forced to jump off when the pirates call, 'land ho', or simply use the spell to make the escape easier, you will not get that chest. Neither of these affect the story.
The 'stone of teleportation' really serves no purpose and is not really all that tied into the story. Considering that the fly/eagle spell essentially serves the same role, and is more useful. its possible for the teleportation stone to randomly drop you into dangerous situations like out at sea, or into the arms of the Abominanable Snowman, or into the hands of the bandits. So its also kinda dangerous, and a time waster (especially if it gets you into places that require you to cast the spell again).
I do agree that the dough, it does improve the story, but its still not needed to understand the game's story. It also requires the player to stand around waiting to listen to random conversations, that may or may not appear, when the player enters a screen. Sometimes the animals talk the moment you enter, sometimes it might take 20-30 minutes to hear their messages. So its 'benefit' to each player varies. Also each animal has about three different messages, and if the player doesn't know about that, they might miss any that are more relevent to the game's plot. Many of the messages serve no purpose other than flavor (having no direct relation to the main story).
Since there is some challenge in finding some of the spell ingredients, it's quite possible to miss them when working through the game. There are other points to be missed in the game as well, that aren't required to beat the game, such as listening on the bandits as a fly, or discovering the badnit's hideout as a fly.
So really most of the spells aren't particurarly important. I know I missed finishing one or more of these spells my first time through.
BTW, if you finish the oracle, and/or defeat Mananannan, before finishing the dough (which is easy to do), it sets off the timer for the pirate ship! You pretty much have to start leaving for Daventry. So if you wander too much, you might end up in one of the game's many dead ends (stuck in Llewdor)! Not everyone knows its a good thing to finish the dough early on, nor does that mean they have found all the ingredients to finish it early on. That timer is related to more than just Manannan.
Another spell in KQ8 is required, the hunt for the three items needed to make the ring of illumination (this is similar to the three item spell in KQ7 to cure the troll curse). That one is tied into the story, and is require to move on.
Also for me personallhy I don't judge KQ8 on the merit of its 'spells' puzzles only. The spells are only one small aspect of that game's puzzles. There are quite a few classic adventure game style inventory puzzles, to use as examples that are vastly superior to anything in KQ3. For example, the very logical, 'cutting down a tree' to stop water. Using a fire to melt ice shard, then using frost to freeze the shard to make a lever. These are clever and realistic puzzles. I also rather like the puzze for defeating the basilisk, that doesn't require directly fighting the monster in the game. Yes, its another hunt for items to make a special weapon. But you still have to figure out how to use that weapon. These are some of my favorite puzzles in the entire KQ series actually.