The website owner is right: Just because you CAN design your own book cover doesn't mean you SHOULD.
The book cover matters a lot.
I can still remember my first Murakami book, the Wind Up Bird Chronicle. I was browsing at the bookstore, and the cover caught my eye:
And so I flipped through it, read the first page, and immediately knew I had to buy it. But if the cover had been one below, then I probably wouldn't have noticed it:
It's not terrible, but it's not eye-catching. The ones with half-naked Asian women on the covers are the worst - I wonder who approved them as the book cover in the first place?
So yes, cover art is important.