I would argue that anything with a narrative structure more complex than a short improv sketch really does need a solid script.
Improv sketches follow a narrative structure. But that's beside the point.
As for feature films, if you take it one scene at a time (which forces you to take extra care in crafting a great scene every chance you get) and every actor has training in improvisation (which is simultaneously the best acting AND writing training you can receive) you would be surprised with the results. At the very least, it will be
on par with a movie that had a decent script, provided that you edit well.
the script is there to provide structure and direction.
Go watch an improv show. If you think you need a script for this, then You'd be
amazed at what you see. I could provide you with descriptions of the techniques if you have any questions. (for the record if you do watch something to see this in action, no shortform shit like Whose Line Is It Anyway. that's not true improv.)
Do you have any examples where scripting held back creativity?
Every bad movie that has ever been made. Especially when the people making the movie
knew it was bad.
Do you believe the odds of making a great film are high without a good or great script?
If the creative team and actors
(I guess this would make the actors part of the creative team) know what they are doing, knows the genuine process of improv, not just "winging it" or whatever the uninitiated may think improv is, then I think the odds are higher.
but does that mean you go into the project thinking "You don't need a great chassis to build a great car"?
I didn't say
go in without an idea. That's not what improv is.
Improvisation doesn't mean you don't have any material to work with.
In Improv, you're using a very specific creative process, and it works very well in comedy, which is ultimately more difficult to pull off well than any other genre. Hell, Improv Drama has become popular over the past few years and, using the same process for comedy, provides great dramatic results, with much less effort.
There is no reason it wouldn't be beneficial in the hands of a creative team that knows what they're doing.
"You don't need a great chassis to build a great car"?
Think of it like this: 'you have a great chassis, you want to build a great car. Either you learn to build great cars and build this with love, attention to detail, and a proven technique,
OR you can use this step-by-step lego construction guide and
maybe it'll turn out good. '