The phase-detect auto focus is based on comparing light rays reaching the focus sensor from two facing sides. The smaller the maximum aperture, the higher the chance that light reaches a peripheral focus sensor only from one side. Thus, dead center of image is the sweet spot for an auto-focusing sensor to be as it will receive enough light from two sides in the biggest range of apertures. Finally (somewhere around f/6.3...f/9), though, the directions will be so subtly different that focus cannot be determined even in the center.
To conclude, being bunched in the center is good because more AF sensors will work with a wider range of max apertures. For people that compare cameras by numbers (x AF sensors work at aperture f/y), and/or plan to use slow lenses (or fast lenses with tele-converters), this looks good. You'll also appreciate the higher sensitivity and smaller gaps between focus points when tracking fast-moving objects; in order to avoid accidentally cropping them, it's a good idea to keep them near the center of frame anyway.
But it also means that phase-detect AF by points further away is not possible, so you are forced to choose a workaround, all of which have some kind of downside:
- place the subject nearer to center, possibly resulting in weaker composition;
- use the "focus and recompose" technique, and either accept the ever-present focus error caused by recomposing, or make your best guess to compensate manually;
- use contrast-based focusing in Live View, which is very accurate, but just plain slower and forces you to give up stabilizing the camera by holding it against your face;
- shoot wider than necessary and crop in post-production into desired framing, losing some resolution;
- use that big full-frame sensor in DX crop mode, effectively cropping peripheral areas not covered by AF sensor, losing roughly half of pixels.
With a slow lens or shooting stopped down, the second option (recomposing and ignoring) is usually pretty much okay. For moving subjects, you'll have some extra focusing inaccuracy caused by subject moving away from metered location while you're recomposing. Laying out cash for a full-frame camera, you might be intending to use some fast lenses to get that oh so thin depth of field, and that's where you would notice the focus being slightly off after recomposing; so in this case, having the AF sensors flocked up in the middle is bad.
Note that designing an AF system with widely spread focus sensors has been rather easy for APS-C DSLRs, since the all crop DSLRs so far use flange distance and rear element diameter dictated by lens mounts originally designed for a larger format (the full frame). Full-frame cameras don't have such extra room to spare, and getting light from the edge of lens is more complicated.