As someone who is new to LyX and completely new to Latex I couldn't agree more with Daniel. But I also agree with Charles Stewart. The point is rather than these things being mutually exclusive they are actually interdependant or mutally supporting.
I write technical training manuals which are used both in the classroom and as a reference afterwards. When writing a new course we have to identify prioritize and clarify lots of interelated areas; technical content, scope, levels of detail, learning objectives, skills objectives, presentation, clarity, conciseness/economy, consolidating practicals, course timings, classroom environment, technical build etc etc. All of these issues need to be reflected in the final document.
I'm sure nobody is going to suggest one would write this off the top of their head directly in LaTeX. Our experience suggests that to develop the basic structure of the course/manual a good outliner is absolutely essential as a minimum. A mind mapping tool can also be very useful.
LyX has a pretty good Outliner
Once you have the basic outline structure you have to develop the subsections and bullet points into paragraphs, pages, section and chapters.
For this you need the basic document formating tools which LyX also has.
However while we're developing the 'text' component of the manual we also have to think about how the content will be presented live in the classroom. Hence we design some of the diagrams for the slides early in the process and write the text content around the slide content. Consequently the slide/graphic development proceeds hand in hand with the textual content. LyX has great (LaTeX) tools for both text and slide development.
Finally we proof and adjust the content, length, layout and graphics of the document. Here the LaTeX underpinning of LyX really comes into it's own.
A single tool which allows one to progress from high level general outline down to detailed print layout and which takes care of much of the details for you is a really powerful concept.
We simply wouldn't consider using LaTeX by itself to develop our manauls simply because it is too low level for us. Nor would we consider using LyX if it didn't have the LaTeX underpinning. Although we feel that the WYSIWYM is a better paradigm than WYSIWYG, LyX without LaTeX would not offer enough for us to migrate from using two tools; Word and Impress to using one tool; LyX.
Is LyX a LaTeX editor? Probably not but who cares! Many people probably don't use a LaTeX editor because they like writing LaTeX but because they want the great quality and control it offers. It's the great output that matters and the time and effort it takes to achieve it. Any tool that makes that process easier for a given use case is to be praised and recommended.