I can't speak to the Airs, except for reading about them, but I can compare the MBP and MB.
I really like white plastic laptops, aesthetically, and I'm used to them, having had an "icebook," and then a non-unibody plastic Macbook. When the latter Macbook died prematurely, I got an MB under warranty. I was glad to see the plastic one had gone unibody, as I'd had quite a few hairline cracking and delamination issues with my non-unibody one.
But then I found out that a couple of the features I really liked on my former MB had been *removed* from the new MB. These were Firewire (I don't use it daily, but I did NOT want to give up the option of Target Disc mode), and the external battery monitor. Heck, both of those were on my early 2000's iBook!
The external battery monitor is a small feature, but one I use quite often. I tend to keep my computer in a backpack and/or shut down or asleep, and I like to be able to just press a button to see how the battery is doing without opening, waking up, or starting up the computer. Of course you may not need this - I'm just speaking from my point of view. And of course there is always something "worse" about having features "taken away" when you already had them in a previous version!
One other small thing that was on my older MB but has now been taken away: The infrared port so you can use a remote. That can be nice for slide shows, music, etc.
Here are a those things on a list, as well as few other differences I noted in the MBP (i.e. things that are not on the MB at all):
1) Firewire port
2) External battery monitor
3) Infrared port for remote control
4) The 4g RAM, obviously, and capacity for more total RAM (I didin't know you could "force" the MB to use more RAM as someone mentioned previously in this thread).
5) Better speakers (subwoofer, I believe)
6) Backlit keyboard (which can be turned off sometimes or all the time, if one so chooses; I am liking it because although I can touch type, sometimes when you are web surfing/chatting in bed or on the couch, you don't "know" where all the keys are.
7) SD card slot (wow, this is handy!)
8) Black bezel (this didn't matter to me, but just mentioning it).
All that said, from what I understand, the current MB and 13" MBP use the same screen/display/glass (someone commented that they were different).
The above differences were enough to make me go MBP (luckily I had a friend on the verge of buying a refurbished MB, so we struck a deal). Additionally, there are authorized Apple resellers in the US who sell the MBP for $999, so some people don't even have the $200 difference (I have not noticed the MB for an equivalent reduction).
I'm not a huge fan of the aluminum, and I don't like the edges on either the MB or MBP (too sharp to my mind). But until they change the design, or until the Air becomes the computer for me (currently the Air I would have chosen would have been nearly double the MBP so I'm holding off), well, I'm enjoying the MBP

I bought a Moshi Palmguard to keep the "humanity" off of the anodizing

If that's not enough to quell the edge, I may get a Speck case and then round the edge of that with some wet/dry sand paper. So far that's not needed.