Hey Kevin,
Thanks for the question! No doubt many arguments are flawed, but not all of them are, and in the battle vs the LSAT, every piece of information is beneficial. In those instances where the argument is not flawed, you gain an advantage on the test makers by knowing this fact. For example, non-flawed arguments immediately suggest certain question types (Must, Method, Parallel, etc), and that information can also be used to your advantage in specific situations such as eliminating certain answer choices in Parallel questions that contain flaws. The rule is: if the LSAT gives you a handhold of any sort, take it and don't look back!
In its final form, the validity analysis should eventually take no time at all and should instead be a built-in part of your overall breakdown of every argument. And keep in mind that while I separate everything out into discrete steps in order to make learning it all easier, that is
not how it works on the LSAT. There, everything blends together seamlessly and becomes a smooth process that is applied all at once. Think of it as learning to tie your shoe: when you start you do each step slowly and somewhat laboriously, but after a while it becomes a fast, effortless process. Now, most people would have to stop and go through each step if they were asked to describe the process. Those learning the LSAT are at the start, but eventually—with enough work—they get to a point where they can make all these analyses without thinking
Please let me know if that helps. Thanks!