Friedl's argument from the above post is true enough:
Although it's a stretch that there are a "myriad" of apps that support layers.
To be sure, the Perfect Layers app is just a plug-in that has a layers function but without the functionality of Photoshop, Elements, etc -- in that sense if you have one of those apps and want layers then you get more by using them.
And, his point about "destructive" editing is certainly true in the sense that like all plug-ins it creates a psd/tiff that you are then editing and when you save the file, well, you save the edited file although the layers are intact. In my little bit of testing I didn't try opening a file in Photoshop to see how you might be able to work with them, though, and I also didn't save and re-open a file in Perfect Layers to see if you could revisit and adjust those layers.
So, all that being said, will this be useful? Like was said, if you have an editing app that supports layers, the only thing you might like about it is that it's "small" -- the interface is like a mini-layers palette without all the overhead of a full-bore editing app and that could be appealing. On the downside is the limited functionality, meaning that if you really need to work over something you'll still need to resort to PS or whatever.
As to whether this will in any way infringe on the PS market, well, I'd say nah. Lightroom itself has certainly become more of a go-to app for many of us, but we still have either PS or another app on hand.