[...] And then you only have an average graphics card in that machine.
... which can be exchanged on a Mac Pro, but not on an iMac!
[...] And then you only have an average graphics card in that machine. And then you would still have an operating system on the Mac for which still nobody except Valve writes games.
... and Blizzard. With Valve's "Steam" you get quite some game selection already - others will probably follow, as the Mac continues to gain market share (and the drivers improve - Valve is blazing the trail here).
[...] Guess why at the end of the day I decided to sell my Mac Pro and went with a 27" iMac instead - it's a MUCH better deal.
Might be the better deal for you, but not necessarily for everyone!
With a Mac Pro you can easily add better graphic later - the iMac has to be replaced every 2 years or so in order to get up-to-date graphics (and more modern ports and other technology btw.). My 2006 Mac Pro runs on a still decent 4870 (and the 5870 is already running in some other Mac Pro's) - the 2006 iMac offers a (comparably pathetic) Radeon X1600 and is not upgradeable in that department.
You also have far less clutter on your desk: With an iMac, you probably will need to add external harddrive boxes sooner or later, which not only add clutter, but also do cost quite a buck (e.g. a drobo FS as GB-Lan NAS is around 600,- w/o harddrives). With a Mac Pro you can add 4x 3,5" harddrives PLUS you have two 5,25" bays with at least one to spare, so you could e.g. add a special frame there for additional 4x 2,5" harddrives. All still be powered by the one Mac Pro PSU, whereas external solutions for the iMac do require (external) Plug-in-PSU's, one per external box - yet more clutter, more wasted energy.
Or you can easily add any aftermarket SSD to the Mac Pro (i.e. the one that fits your personal ratio for capacity to money best) in any of the harddrive bays in the Mac Pro - there are lots of adapters for putting one or two 2,5" drives into one 3,5" bay.
On an iMac you better ordered the (expensive!) SSD option from Apple, as you would need to remove the display on the machine, if you'd wanted to add it afterwards, running high risk to get dust into the display while doing so. And even then you would have to use a kludge (like removing the optical drive or replacing the factory-installed harddrive), as there is no dedicated bay for the SSD.
And because you may need to add a SATA controller for additional SATA ports after adding more and more drives to the Mac Pro, you can easily do so by simply putting a SATA card into one of the Mac Pro's PCIe slots. The same way you can also easily add eSata and USB3 to the Mac Pro if you feel the need to do so - Apple won't built you such an iMac at all! Your best bet on the iMac would be external adapters, which not only add yet more clutter, but also don't offer the speed of "native" ports (considering PCIe-based solutions as "native" here as well).
Also noisewise the Mac Pro is one of the quietest computers out there, in relation to its capabilities. Granted - it may be louder than the iMac in direct comparison. But on an absolute scale it still is unobtrusive and really whisperquiet when put under the desk. And as soon as the iMac out-of-the-box is not sufficient anymore (e.g. HD too small => need for external HD space), things may start to change even more.
So in my opinion there are quite some reasons that make the Mac Pro the better deal compared to an iMac, even with games in mind. The 2006 Mac Pro had a fantastic price/capabilities/performance ratio (even more so as refurb), which Apple realized and thus upped the price to come closer to the "usual price point" of the competition in that market.
Unfortunately this also opens the gap between iMac and the often demanded "headless Mac" even more!
Imagine a shrunk Mac Pro still with room for expansion (2,5" instead of 3,5" drive bays or two 3,5" bays with the option to put two 2,5" drives in each of that bays, 2-3 PCIe slots, user-changeable graphic card etc.) and less expensive components (e.g. desktop class CPU's instead of server-class), while still maintaining the quality and internal clean structure of the Big Box Mac Pro. Due to the restrictions (less HD bays and PCIe slots than the Mac Pro, "only" desktop-class components) it would not bite too much into the Mac Pro's sales numbers, but instead allow Apple to sell additional units (people who don't want the iMac for the reasons i described above, but can't afford or justify the expenses for a fully blown server-class workstation).
People often state that Mac users would not be the DIY type - but "internal expansion" is also about avoiding clutter (something Apple aggressively advertised with the iMac) and expanding slowly over time, as money allows, technology advances and requirements demand: One of the key success factors of the first successful Mac - the Mac ][ - was the presence of expansion slots!
And it's about prolonging computer lifetime - an iMac is quite obsolete after some 3 years, while a big box MacPro will easily live twice the time. So the TCO (
Total
Cost of
Ownership) might not be worse (if not better) for a Mac Pro over an iMac...