That's undoubtedly true.
But there's more to portability than weight - it's the combination of form factor and lighter weight. I travel constantly, and it surprised me how much more convenient the UMB is compared to my previous-gen MBP - but it is. Takes less room in my bag, weighs less (which I notice over my shoulder running / jogging / walking through an airport) and is just easier to use in the field to me - on a plane, at the airport, on a lap, or on a table in a Panera / Starbucks / Insert restaurant name here.
Sounds like your trying to conyourself into buying the cheeper model. Thats fine. If your a traveling bible salesman or whatever you probably dont need the abilitys of the macbook pro.