Why buy an i5? it's just a slower i7!
Price/performance. You get a mild speed bump in compute power on a 4 core nMP vs 6 core oMP which costs literally twice as much. Do you get what I'm saying? Twice as much is a boatload of money. As far as upgrades, I'd hardly call throwing in a few cards and some RAM "tinkering."
CPU upgrades are a pain, I'll agree with you there. None of the rest is really "tinkering." It literally takes 5 minutes to upgrade the Old Mac Pro.
Really, if you're not using a ton of OpenCL (or firePro specific apps in Windows), the nMP's price/performance is abysmal compared to what's out there. Apple tricks people like you into thinking things are a universally good deal because some people are willing to pay for them. Guess what? this guy doesn't want to pay $2000-3000 extra for a machine to run FCPX 40% faster--which he barely uses. That's like saying somebody needs a $100,000 dump truck to commute to work.
That's not to mention the real price of the nMP. After all, how much do TB HD controllers run in comparison to throwing another drive in the drive bay? I love how people are obsessed with resale value too, because that's such a lame counterargument for the fact that you literally have to sell the thing in order to upgrade the GPU (and CPU, if you want the next socket)
Also, the video cards are already 2 years old with no promise (and little likelihood) of upgradability. You're buying old technology! Call the President!
Thing is that most of my 4/5,1 users don't care about old GPU's, tock generation Xeon, they just want a new Mac Pro. Companies in media, broadcast and design. The finance has been paid off, tax deducted, most have had my hands inside their cheesegraters with CPU GPU SSD upgrades but they still want a new one. I've seen a hex core nMP sitting next to dual hex cMP with all my upgrades only last week and as far as single core, booting OSX and apps and all round snappiness the nMP is on a different level altogether and so is the lack of noise even with the nMP + Drobo 5D combo.
I'm actually quite happy they want the can cos I'm rubbing my hands at the prospect of getting their cast-offs on the cheap and one under my desk will certainly do for my upgrade. They will be writing all their new cans and TB spaghetti off against tax and will not care thankfully. Though after their upgrade experiences with the towers they may let me drop in upgraded Xeons in the future, perhaps more even daring to do so before AppleCare runs out!