Now, I'm never going to tell another writer, "You have to use an outline" if they work better without one. If someone can get a novel done without an outline, excellent. I'm a big believer that every writer should do what works for them. But I do raise my eyebrow at one of the big arguments against.
Namely, "If I outline it, then I feel constrained in the writing."
I've never found this to actually be the case. In fact, I've always found wonderful discoveries in the writing process. Case in point, on the currently-in-rough-draft USS Banshee, I have not strayed from the outline, yet... but I find a fascinating thing I didn't expect. I added an extra element of danger to the initial action sequence-- nothing really odd, but not something I planned. But that element, and its effect on Lt. Kengle, is important... and it makes it necessary to make certain changes to how Kengle first boards the Banshee. Now, again, this is nothing that technically goes off outline... but in the outline, there wasn't much to it besides, "Kengle boards the ship." This new layer makes that scene a moment, and (if I'm doing things right) gives more depth and understanding of the character.
Those are the things people fear outlining will strip them of. And I can tell you, it doesn't happen that way.