Last i checked, 3d modelling wasn't a common (read what I wrote) use for portable macs - and if you need that, there's the 15" option anyway. Its only a matter of time before thunderbolt GPUs are available anyhow.
I did read what you wrote: "most professional purposes that the mac is used for". 3D modeling is most certainly a common professional purpose that the Mac is used for. Once I made that point, you retroactively stated it to mean "portable Macs", but that's not what you said. You said "the Mac" which is Macs in general. I answered what you said. If you meant something different, be more clear in the future.
Your revision is kind of self-answering. Professional uses I would use a 13" MBP for exclude everything the 13" MBP isn't good at, thus it self-defines as a perfectly fine professional machine.
The 13" pro machine is fine for various pro-level activities that do not require 3d, such as DJ-ing, music creation, virtualization, photoshop (sure, the display is a bit crap for that, but externals work), etc. There is very little benefit to a professional musician (for example) for a 15" pro over a 13" pro. Trying to use an MBA for that task however would be a joke. Hence, the 13" is a valid professional machine, within the apple lineup.
It would work fine for DJ-ing. Music creation, virtualization, and photoshop are all OK for light to medium use. I would never consider, for example, trying to work on a 4 GB Photoshop file on a 13" MBP if I had a better option at hand. Virtualization is OK as long as you're not doing something intensive in it, but would benefit greatly from a quad-core processor.
The 15" and 17" machines don't have replacable CPU or GPU either, I really think you're clutching at straws here.
You're absolutely right that they don't have replaceable CPUs and GPUs, and that's one reason why a lot of professionals pass them up. It's one reason why many people don't consider ANY of the MBPs to be true professional machines. I'm not really clutching at straws, for some lines of work it's a requirement.
You'll always find "pro" tasks that can't be done by "pro" machine X. The MBP 17 can't take 96gb of ram and only has 4 cores, not 12, so maybe it shouldn't get a pro label either (because the Mac Pro can do that)? All of your arguments barring the GPU apply equally to the MBP 15 and 17, the screen res on the higher end 15" is pretty crap (compared to say, the Dell E6500 i had at work with 1920x1200 3 years ago), too.
The hi-res option of the 15" takes care of the screen res being crap on the 15". You're right that the term "pro" is relative, but it does have a bottom cutoff. IMO, the 13" MBP doesn't meet that bottom end cutoff. TBH, it would if it had a discrete GPU (though I don't know how they would fit it).
Fact is, the MBP 13 is a lot more capable than a 13" air, which is why it is branded as a "Pro" machine, and the MBA isn't.
If you want to talk about facts, the performance difference between the current 13" MBP and 13" MBA is less than 20%. That's not really "a lot" in my book. It's got quite a bit more in terms of expansion, though.
As to why it's branded as a Pro machine? It sells better! That's really the primary reason. My MBP is nearly identical to the aluminum MacBook before it. What makes mine a Pro and that aluminum MacBook not? Other than Firewire (which the MacBooks all had until the aluminum MacBook), there's very little difference. It's completely a branding thing.
"pro" is a relative term. doesn't it depend on WHAT you do professionally?
Yes. But if you get paid to write stuff in Word all day, then technically ANY laptop would be a professional machine.
if you're doing pro work and feel that the 13" won't support it, you can ALWAYS get another mac (or a pc, whatever). stop whining and find a computer that lives up to your needs...
How many times do I need to say that I have a 13" MBP and I love it
It works for what I need it for, but that doesn't mean I consider it to be a professional level machine. One of the reasons it works so well for what I do is because I have an external monitor. Without that, I would be hard-pressed to do everything as well.