KenGrinsven :
The problem with PSUs is usually QUALITY rather than quantity - even high-end single-GPU systems rarely draw more than 400W. The recommendation for 500+W is mostly due to the large number of sub-par PSUs in the field and on the market.
Indeed, I think everyone understands your point. The problem is, if for some reason there's a spike in wattage consumption, the 430W PSU, no matter how good it is, will have a hard time, possibly causing a crash.