First, level the playing field. Aside from the display and about $79 of third-party RAM, the specs of that Dell $1399 system are closer the specs of a more portable $1499 MacBook.
Second, it's Dell who's double-charging. This $1399 Dell is a Windows machine, period. That's fine if you're okay with just one machine running a copycat wannabe OS. But on the $1499-$1578 MacBook, add $80 of Parallels or VMWare Fusion and an OS from a dead PC, and you've got
both a Windows machine
and a Mac OS X machine, at the same time, in one little box. Windows virtualization exists, but it can't run a real Mac OS, only other flavors of Windows or get-what-you-pay-for freebies. Whee. Not.
If you really want justification to buy a MacBook Pro costing double that Dell ($1399 x 2 = $2798), you'd also have to be able to justify buying a $2798-ish Dell as posted by others. If you don't need anything more than the $1399 Dell, then you probably also don't need anything more than the $1499 MacBook.
Now, if the extra 2.1 inches of screen space and more powerful graphics card with a 160 GB HD are really your deal-breakers, then you make a good point that the $1000 difference between the $1499 MacBook and $2499 MBP is excessive for your purposes. Simplified product lines do that to us. Back when the MacBook didn't exist yet, and the specs I needed in an MBP cost less in a 17-incher than in a 15-incher, I chose to go with a bulkier machine than I really wanted in order to save a few hundred bucks.
These are first world problems.
For example, if I can afford a $2798 MBP, why not a combo of MacBook Air ($1799) with a Mac Mini ($799) and 19" LCD ($150 on pricewatch), all for $2748 ($50 less)? I get 200 GB of HD space, four cores, a superdrive, firewire, ethernet, and a big monitor, but split among two units. With Back-To-My-Mac, I could leave the Mini at home on trips; or if I need Firewire, Ethernet, or a Superdrive once I get to a hotel, the Mini is small and sturdy enough for checked luggage, if not the extra space left by the Air in my old Pismo carry-on. With Remote Desktop, the Air can act as KVM for the Mini, so it only needs a power cord. For hotels with Ethernet but no WiFi, the Mini can act as a base station for the Air. Then, there's the whole backup issue: "Got me an Air and a Mini, too; when one don't work, the other do." But, the MBP has a better graphics card.
Oh, the dilemma!!