Yes, focus does matter a lot, or you might regard this as a Post concerning all Matters About Focussing. Either way is right.
I love the above picture for it's composition and creative use of exposure. However, there are some focus problems within it. The waitress and the two ladies seated at the very back are not in focus, and this really shows up on a print that is 8X10 or bigger. It sort of doesn't matter, because of the low exposure, and emphasis on the suspended lamps, but it does matter because it is somewhat distracting.
This was a case where I let the camera's AF make the decision, and I wasn't careful enough to make sure the AF was doing the right thing. In fact, I honestly wasn't thinking about focus at all, because I was so absorbed with getting the correct "moody" exposure, I ended up taking "Auto Focus" for granted, and I don't even know for sure what the camera did, or how I had the AF set, but from looking at the picture, it would appear that I was using the default Multi-Point AF, and the camera set itself to provide best focus on one of the lower points, giving very good definition in the foreground, and poor focus in the background. At the very least, I should have manually selected a more appropriate AF point, along with a deeper (smaller, higher number) Aperture value. I highlighted those words for a reason - it always strikes me funny how that with "automatic" cameras, we still have to be careful to manually set up the right automation to best suit the picture! It's not nearly as easy as putting your automatic transmission in reverse when you want to go backwards! This is what makes me a fan of older style manual lenses, and classic film cameras.
We must be very careful about how we use Auto Focus, and that it can be in fact, just as laborious and time consuming as manual focus. Personally, I prefer using manual focus, simply because it's much easier than trying to select one of the 9 (or in cases with "better" cameras, 19 or more!) AF points using the camera's push-buttons and control dials. With a manual focus lens, you simply focus on the main object of interest (which is not necessarily in the centre of the picture - in this case it should've been the waitress), then re-compose, and finally stop down the lens and press your Depth of Field Preview button to check on what's actually in focus from front to back.
You can use a similar approach with auto-focus by setting up your camera with it's "non-default" single- centre focus point, focus on the object of interest, half press the shutter - the camera will respond by automatically setting the focus AND the exposure, then re-compose, and press the DOF button to check your front to back focus. But be careful - now the exposure may not be what you want, because the exposure automatically got set at the same time the focus did! If I had done this by centring on the waitress, the whole picture would've been auto-exposed much brighter, because the camera would've compensated for the waitress being in shadow. Further manual effort would then be required to lower the Exposure Value (EV).
This should serve to explain why modern DSLR's have all those buttons and dials on them - yes you can re-program your camera using these buttons and dials to separate the focus point from the exposure point (such as by pressing the "exposure lock" button), and then focussing, or you can program it to do the opposite - program that same button to be a "focus lock" and then create the exposure you want, before taking the picture. But then, don't forget to change everything back to "normal" before you take the next picture. Personally, it all makes my head spin, and it's all a perfect example of how the camera can get in your way sometimes.
Here's a good article that describes the "old" way of focussing a camera, using the Depth of Field scale on the lens barrel (something that's absent from almost all modern "digital" lenses). It may sound complicated if all you are used to is Auto-Focus, but in actual fact, for tricky situations such as I encountered with the picture above, this would have been much easier, and more of what I'm personally accustomed to.
Aside from all that, no matter what camera you've got, the most important thing is to get to know it. Learn what all the buttons and dials do, and practice with lot's of pictures. Then form your own opinions about the way of using the camera that works best for you in every situation.