We have been using Mac Pro computers and are being asked to consider mini or iMacs.
The only real reasons to stick to a Mac Pro:
If you aren't using any of the above items, then a mini or iMac would work just fine. The Minis and iMac have plenty of processing power for most design work.
The Minis can use up to 16GB of RAM and iMacs can go up to 32GBs of RAM. External hard drives can be daisy chained for use (Firewire or Thunderbolt) as well.
The only time I would strictly suggest sticking with a MacPro is if video editing is common. The additional cores of a MacPro really speed video processing much more than Minis or iMacs are capable of. But this really only means time spent waiting. The Minis and iMacs can certainly edit video, it just takes a bit longer.