A: People who want expandability.
Some people want expandability, but all users are different.
Some users just need a ton of processing power but will never use the PCI slots at all. Other users do need a ton of ram.
Actually you're incorrect. Since the MacPro is the only mac that offers multiple open drive bays. People who want expandability but not your narrow definition of that (ram) will want the macpro as well.
Just because you fail to see the need for multiple drives, or large storage, or a RAID array does not invalidate the other person's requirements.
Multiple drives and large storage, even raid, can be done externally via FW, USB, or even network storage via ethernet. Sure, if you need really fast storage, the MP is the way to go. But the example given was lots of music and movies, which will work just fine on external storage on ANY mac.
Someone pointed out, you can even do storage wirelessly. I do that with my mini and it works fine.
the iMac fails the cheapness test over time. the screen kills you when it dies. you can only warranty the screen for 3 years. every part in a macpro i can replace that means it lasts far longer. if I use the imac fw800 is slow in comparison to esata. here is the big kicker if the imac hdd dies your have to bring it to the shop it is far too hard for 90 percent of the public the swap the hdd out... if my mac pro hdd dies less then 1 minute and it is swapped... the closest thing to the mac pro is the 17 inch MBP it is 2299 the mac pro is 2499. I prefer the mac pro 1 it has a better faster cpu A 2.8 QUAD. mbp has a 2.53 quad.. the mac pro can handle far more ram 32gb vs 8gb. I can drop in 8tb in hdds in a minute. run them separately as jbods or lots of other ways. I can add a second dvd drive. apple care for the mbp is 350 for the mac pro is 250.
If the screen dies you can just pay to get it repaired or even use an external monitor instead. If you have a hardware failure in a MP you're still going to have to repair that, if it's something like the motherboard it will be extremely expensive. eSATA is irrelevant for storage of movies and music - are you going to be streaming thousands of songs at one time? Or copying your entire library from drive to drive on a regular basis?
I'm not a fan of the imac, but for someone who just wants to store lots of music and movies it's hard to buy the notion that a MP is somehow "cheaper" for that use.
By the way, the higher end iMac supports 16 gigs of ram (I assume that it can't use the 8 gig chips for 32 total?).