Even the portability is not necessarily a win for the MBA depending on your preferences. The footprint of a MBA is slightly larger and the depth at the max is essentially the same (I know the MBA tapers way down, but that does nothing for me.). The biggest advantage is the weight of the MBA is 0.5 lbs. less. No way would 0.5 lb. weight savings be worth it to me for a significant performance downgrade.
I just went through this as I was looking at buying a 2nd backup machine to keep around the house and leisure travel (light duty - surfing, email, some photos, video watching, maybe light word processing or simple spreadsheet work). I assumed the MBA would be fine, but once I looked closer, the relatively small amount of $$ differential was well worth it to me to not have to compromise.