As a landscape photographer, it's important to me that lenses be of good quality, right to the corners. As I use a 16 MP camera, any flaws in the lens tend to show, should I some day go to an even better camera (say the 1Ds4) then it will be an even bigger issue.
That said, 95% of the images I take are shot at f11 or f16, perhaps 0.5% at 5.6 or wider.
For me, looking at resolution figures for wide open are pretty pointless. Looking at peak resolution, often at around f 5.6 isn't really all that much help either.
More practical is looking at f8 to f16 since those are the working f stops.
Other types of photography have different requirements. Someone shooting theatre or sports might well need maximum resolution wide open. They may well however not care much about resolution in the corners where I obcess about it.
Some lenses that perform very poorly in the corners do so because of a focus shift (ie. not a flat field) rather than simple outright low resolution - the 17-40 L Canon lens is known for this, focusing closer in the corners than in the middle. Stopping down and adding depth of field may well bring the corners within the area of decent sharpness.
So, as you can see, it's important to 'read' resolution charts and graphs and tests with your own needs in mind.
You'd think that once you stop down pretty far, all lenses tend to be equal. While there is some truth to this, diffraction being what it is; the reality is that complex lenses don't follow simple rules and diffraction does in fact vary from lens to lens and it can be important to check lens performance stopped well down.
That said, you can sure see what happens to resolution when you really stop down - say f 32 on a full frame camera or even f11 on a thumbnail sized sensor on a point and shoot.
Lens flaws like barrel distortion and even chromatic aberration are getting easier to correct, even with Photoshop and Camera Raw so are less of an issue than they were once. Sometiems though the distortions in a wide angle zoom are complex, featuring both barrel distortion in the middle and pin cushion at the outsides (as the lens designer tries to compensate for the barrel distortion) and these complex distortions require outside help from PTLens or DXO etc..
Long and short, consider your needs before comparing two lenses - if lens A is dramatically better than Lens B but only in ways that aren't useful to you, you may be picking the wrong lens, paying too much or carrying too much weight.