I find the auto-preview useful when I'm making tables or figures. It also enables some cool stuff, like this 'workbook'-like document that I recently did with a friend:
http://writelatex.com/4134bzfwng
There should be probably be a way to turn off the preview, though.
It doesn't necessarily, but I find myself spending too much time tweaking visual appearance and layout if I compile too often. I prefer (if I can manage the self-control) to write up a semantically oriented LaTeX document and then only do a round of layout-tweaking (e.g. babying the figure-placement algorithm) at the end.
I really wish there were Latex editors that would preview just the equation you're currently editing. I know macros make that tricky, but it would be a killer feature for anyone getting started with Latex.
I believe that AUCTeX [1] does essentially this in EMACS. It embeds a preview of each equation into the buffer, but I don't know whether it's smart enough to handle macros.
