Nope, you don't need Knights of the Nine to install Shivering Isle.
Though Knights of the Nine is not bad either... just average. The KotN quest involved many dungeon adventures and new kind of quests (problem solving) in recovering the artifacts.
And some of the official mods bundled in it are great.
Frostcrag Spire
Pros:
Looks amazing
The largest amount of storage area in the official homes
Garden that respawns alchemy ingredients (including ones from Oblivion)
Alchemy table that really boosts your skill
Cons:
No quests
Since my character is a goody-two-shoes, I haven't checked out the other two homes yet, but I heard they have a few quests. One is for thieves (including a traning area, merchants, and the rare fortify skill spells), the other is for vampire (a prisoner for all your bloodsucking needs, etc.).
Horse armor looks good, and gives your horses a boost to their health.
Mehrunes Razor itself is average, but in the process of the quest you gain access to a huge area which quickly respawns, and a large number of other interesting loot (new magic items).
And unlike SI, KotN introduces no new bugs (okay, some, but not as much as vanilla or SI)... you should consider it when it gets cheaper.
