The storyline is the most important thing...
...that would be true, if you were writing a book.
Storylines have an important role in roleplays too...
...but they are not completely nessecary.
I believe you know the thread "The Tavern"....That roleplay has no storyline whatsever, yet it was once the most popular thread out there.
Here, this thread...say someone wanted a thread with pimps and werewolves, we could use and build upon it.
...or...
We could make a thread, with just pimps and werewolves and let the story go where it wants to go.
A storyline offers more control over a Roleplay, though it's not completely nessecary.
You do have a point there on the storyline being not necessary due to its "using and building upon it as it goes" functions. It'll enable people's imagination in order to use it and spread it over the horizon without any limits, but it's not common in the roleplaying community. Why? I can tell that it's quite common in here, and I just wanted to offer my two cents by telling that the storylines do exist in roleplayings as well.
Coming from my perspective based on my experiences with roleplayings on foundations, I cannot imagine roleplaying in a roleplay without any the storyline. Oh, how do I explain this? Let's say, if someone hands you all pieces you need to build an object and they don't have any instruction on how to assembly these pieces together, and now you have to do it with your knowledge. I feel it's a lost cause because you don't know what you can do with these pieces without any instruction. It goes same for the roleplaying without a storyline. The storyline is like an instruction that you can use outside of the box, limited or not, so you will know what you can do.
-- Grey