Of course, it's true that there are some good pre-built deals that will suit people who aren't comfortable with self-builds quite well.
However, that is only a part of the advantage of self-building. Another key advantage is the flexibility it gives with upgrades and the future savings.
For instance, I wouldn't want an Antec 300 case because it's too small to allow for the future upgrades I'd want. I bought a HAF932 for my last build, along with a decent PSU. This means that when I came to upgrade this time around, I just had to buy motherboard, CPU and RAM, which is much cheaper than having to buy a whole new PC. This was £320, and I'll easily recoup £100 or so from my old parts.
I could reuse my CPU cooler too, because I had bought a model with flexible fitting options. DVD drive and HDDs also staying in from my last system.
Now I don't plan on buying a new case at all (ever...?), and I think my PSU will last another 4 or 5 years and probably into my next build. I'll also probably upgrade my gfx card in a year or so, and I know I can do so with no worries because I have a large and well-cooled case and a solid, reliable PSU.
I wouldn't be able to do any of this so easily if I'd bought the system you listed, even if it's not THAT behind mine in specs.
So, I'd argue that self-builds aren't pointless, and aren't just a matter of preference or vanity projects - there are genuine advantages that go beyond just looking at the price of one specific pre-built rig versus its components considered severally.