I am one of those to whom you refer.
As it happened, I was a late convert from film to digital and the launch of the first version of Lightroom coincided with my purchase of a Nikon D80. At that time, it seemed hugely more intuitive than any of the Photoshop varieties or clones to someone steeped in old-style darkroom processing.
I now use it as the hub of all my post-exposure processing. All my Raw files are imported using LR, all my cataloguing, etc is done in it and it is my Raw converter.
Initially I did, indeed, do 90% of my processing wholly in LR. That percentage has gradually dropped as I have become more familiar with processing digital files. I still do everything from within Lightroom and I always take processed images back into LR for finishing and printing (or export as Jpegs if that is what I require for a magazine of competition).
The "plug-ins" that I use from within Lightroom are Photoshop CS6, the Nik suite of products and, more occasionally, some of the Topaz products.
I use layers a lot now in CS6 - not only for compositing but also for a wide range of processing that cannot be done in Lightroom. A lot of that could be done on a single layer in PS but I tend to use duplicate layers so that I get the huge advantage of employing different blend modes and levels of opacity. The tools I use most in PS tend to be the various clone and fill tools, selective blur tools, transform and warp, etc. I tend not to use Photoshop for the things that Lightroom already handles supremely well such as exposure/highlights/shadows, clarity and vibrance, sharpening and noise reduction and the application of my camera and lens profiles.