I was just complaining about PSD the other day. All the web graphics from graphic river are psds, but I work mainly in Fireworks. The difference in just a few things makes a BIG difference in work flow. In Fireworks, if I was mocking up a site and wanted to slap a button in there, I just drag around the whole thing and group it, then copy and paste it onto my design.
In photoshop, if that button is on a page of other buttons, but all I want is one, I've got to make sure to deal with the folder and I can't simply select parts or layers by clicking them. If I want to save out JUST that object, I've got to either delete all the other layer folders and crop it, or I have to make a new window and then drag the folder to the other document and save it.
Part of this is that I got so used to the layers in Fireworks that it's just hard to switch back. It's SO FREEING being able to see an item and click on it and move it or work on it. The layers in PSD feel antiquated in how they work.
Then there's the clunky vector tool that you have to apply layer styles to in order to work with. Again, it feels antiquated in how it works. Sure, I could work and work in psd and get used to it, but I can mock stuff up in Fireworks so much faster. I can deal with multiple objects faster. I can design a series of sliders for a website and put them each on a different page.
Even the slice tool feels easier to deal with (shape overlays, not just a bunch of dotted lines around the document).
PSD feels like a photo editor that added some web tools. Fireworks feels like it was created specifically to make stuff for screen/web exporting and slicing, etc.
I personally don't WANT to use PSD the way I use Fireworks because it simply doesn't work the same way. It would slow me down.
All that to say, you use what is comfortable to you. You may find using PSD is frustrating, or you might love it. I'm really hoping the PSD import in FW CS6 is better than CS5 though, because right now if someone gives me a PSD file I have to do all kinds of work to get thing into FW without losing the look.