For me, and one big reason for keeping my 13" MBP is future drive technology. (And yes, it can have 8GB of ram, too).
SSDs are still in their infancy. They're only going to get better. And you're definitely going to continue to see them usable in 2.5" drive connections. The nice thing about having the machine with a removable standard laptop drive is that you can keep upgrading to better drives. SSDs will gain size, lose cost, gain longer life cycles, etc. Just as Hard Drives did. I expect the cycle to repeat with SSDs. As they do that, my MBP will be right there with it.
SSDs are still in their infancy. They're only going to get better. And you're definitely going to continue to see them usable in 2.5" drive connections. The nice thing about having the machine with a removable standard laptop drive is that you can keep upgrading to better drives. SSDs will gain size, lose cost, gain longer life cycles, etc. Just as Hard Drives did. I expect the cycle to repeat with SSDs. As they do that, my MBP will be right there with it.