They damn well are!
For some bizarre reason, these questions keep popping up over and over again (especially in Europe). It's probably due to a very influential French-inspired aversion to 'teachable' art theories - which is doubly ironic as much of the 'rules' of screenwriting have been taken from French drama models and theories, which were among the strictest anywhere during the 17th and 18th century.
Anyhow, let's define both plot and structure in the broadest of terms.
PLOT = WHAT HAPPENS AND WHY AND TO WHOM
STRUCTURE = THE BEST WAY TO PRESENT THE ELEMENTS OF THE PLOT SO THE AUDIENCE ENJOYS IT TO THE MAXIMUM DEGREE.
So structure is not the death of creativity, a sterile formula, a dictatorial set of 'rules' invented by incompetent money-grubbing Hollywood hucksters. It is part and parcel of all effective storytelling, in whatever genre and whatever style you choose to work.
Compare it to telling a joke. Start off with the punchline, and you won't get a laugh at the end. Stick the punchline in the middle and then continue the narrative, and the audience will be totally confused. Introduce the three characters (a German, a lesbian one-legged penguin and George W. Bush walk into a bar...) at the end of your joke, after the build-up and the punchline, and the audience will only now be able to understand what the hell everything was about. A joke has a certain structure which maximizes the chances of getting a laugh (the desired result).
And if you don't think about structure at all, and don't want to? Well, you'll still have SOME sort of structure in your script - if only because you have a beginning and an end. But it won't help you provoke the desired reaction in the audience (unless you're going for boredom and/or bewilderment).
The same thing goes for plot. As long as there are characters on screen interacting, having experiences and getting into conflicts, there is some sort of plot going on. A truly plotless film might be a woman (let's cast Emma Thompson in this challenging role) sitting on a bench in the park, and eating her sandwiches. Nothing more, nothing less. It's fiction, because we show an actress playing a part (Nora Sandbone, why not). But there's no drama, no interaction, no progression. And absolutely nothing to keep an audience's interest.
So once and for all: yes, you need plot and yes, you need structure, and the sooner we can all accept this, the happier everyone will be.