The way the competition often moves in software, if you wait too long, you have more and more work to do just to catch up.
Let's turn it around. What does Photo Mechanic still do that Lightroom doesn't do? ... that couldn't be done with a plug-in? Now reverse those questions. What other software would you have to buy to match Lightroom functionality if you are a Photo Mechanic user?
A new license for Photo Mechanic is the same price as a new perpetual license for Lightroom. Do the math.
I don't use Photo Mechanic, but I know it's used as an addition to Lightroom by photographers who need speed (sport photographers in particular). When Lightroom imports images, it only shows the image after it has been copied to the destination folder. That makes importing a large number of images very slow, and you can really do a lot of work until everything has been imported (you can't really cull or rate images if you can't see the whole series).
Photo Mechanic shows the images by reading the built-in previews from the memory card. That way, it can show you all images long before they have been copied to the destination folder, giving you the chance to start rating and culling right away. That's why some photographers use it to import images to their hard disk, and then import the culled selection into Lightroom later.
Johan is spot on (are you sure you don't use it? ).
Given how well it works with Lr, it would surprise me if they went to their own catalog. In any case, no evidence of that. As PM has evolved it has taken on some more browser-ish features, which is its strength in addition to ingestion. I got frustrated with Lr's import and after the kerfluffle with the recent import module finally relented and bought PM, and it's been well worth it. And it even helps speed up metadata work post Lr import, since Lr still lags in that area (doing locations is much faster in PM for example, and so is using controlled vocab, scripting, etc).
BTW another really cool tool to use for ingestion is FastRawViewer. It is better for use when culling if you are concerned with say exposure, brackets, etc since the focus is more on the range in the RAW rather than just say how it looks in the preview as in Lr and PM. It too works very symbiotically with Lr. And it's scary fast (unlike On1's browser which I demo'd; in at least its current configuration it is quite slow on my machine). And FastRawViewer is all about using their own RAW conversion. Check it out; they have some interesting comparisons with ACR/Lr previews. Maybe I should have a separate thread about it....