"Sometimes it's interesting to see just how bad bad writing can be." – Joe Gilles, when sitting down to read Norma Desmond's screenplay in Sunset Boulevard.
Gilles hits the nail on the head. It IS interesting, but more importantly, it's also a tool. Bad writing? We CAN use it; we can absolutely learn from it.
Screenwriting only really works well when all parts of the triangle are connected: the writer, the story, and the audience. And when it comes to reading a screenplay, the audience is the reader - you - and if the writer fails to make a connection with you, the screenplay might as well be a stack of blank paper. We write and make movies not simply for ourselves, but for the viewing audience, and that means we must make decisions with that audience in mind. To do otherwise is just plain foolish.
Good screenwriting uses tricks of the trade to engage the audience, allowing us to put the puzzle together, to add it up. Using planting and payoff, creating characters we hope and fear for, and writing scenes such as preparation and aftermath or reversal of expectations are all effective devices used to help tell your story with great script economy while captivating the audience. When the writer accomplishes all this, the reading is easy.
But here's the irony: if the reading is easy, the writing was hard. The screenwriter does his/her job well when we don't notice what he/she has done so well. The read is simply a joy – a bona fide page-turner.
The problem, however, when a script is written well is that it is often hard to understand "the why". You just know it's good. But when the writing is bad, you can learn A LOT because you can see the mistakes. When you're reading a bad script, take an inventory of your reactions. Is it boring? Confusing? Long-winded? Redundant? Is the protagonist unlikable? Are you connected with the story? Answering "the why" of these questions will help you learn what NOT TO DO in your own screenwriting, and understanding that is half the battle.