Why do you think that's the easiest to attack? i think your point is wrong. i acknowledge that i plainly copied the design but that's not an important issue. Their site design is not one of their weaknesses.
I'm not a designer, but as a consumer I think site design is extremely important. If your site looks like it was thrown up hastily or is a fly-by-night operation, I'm definitely less likely to give you my credit card information. Why is it the easiest to attack? Because I think their design is so bad that it wouldn't be hard to dramatically improve, with relatively little effort.