Upgrading is huge, but I suspect is a cause for lower consumption for Apple. Power users, of course, buy new computers every 2-3 years, so Apple likes them just fine.
But for customers like me, I'm not at all a power user. I run Office, Chrome, Mail and the like. No picture/video editing software like a lot of pros on here are doing. So for me, I have a mid-2009 13" mbp, C2D... so for me, why would I buy a new mbp? Believe me, I'd like to, and I have an Apple giftcard that will easily pay for a new mbp... but if I'm not a fan of the current lineup (which I'm not), why would I spend it right now? I could upgrade my RAM to 8gb (which even then is probably unnecessary) and throw in a Samsung 128 SSD, swap my HDD to the ODD bay, get a Super Drive, and probably be good for at least another 2 years maybe, right? Good for me, sure, I don't need to spend $1200+ on a new Apple machine. After all the slowest component in my computer right now is probably the HD (stock 160gb), so an SSD would be sweet.
So it's great for me that I can upgrade this machine over 3 years in and make it last another 2 years probably... but I think it's rather obvious that Apple isn't a fan of this... I suspect they prefer that customers rebuy ASAP and not half a decade later.
I'm guessing this is what is behind Apple's move toward non upgradeable computers... the bottom line and making it even better. I don't buy for a second it's just aesthetics... if anyone thinks a mbp is a fatty laptop, they need to get a grip. I think my laptop is plenty thin, and I think it's form factor is excellent and still better than most laptops on the market.
Aesthetics, however, is a great cover for rendering laptops non upgradeable, thereby reaping higher profits due to higher turnover. I pass no judgment on the tactic, but business is business, and wouldn't ya know it... companies like and continually want to make more money.

JMHO.
Edited to add - And I think a lot of mac users are just like me... students or recent graduates who bought with AppleCare, warranty up so why not upgrade, aren't power users, have an xbox for games, etc., etc., I think you all get my point.