the reason for display port is because the card can only have 2 total video outputs through VGA/HDMI/DVI. The use of DP enables the use of more outputs.
It was not a decision of marketing, a simple hardware limitation caused them to use it. Nvidia skipped it and only enables triple monitors through the use of SLI. 2 cards can put out 4 outputs without DP.
Yes DP is used for the "Third" of the three monitors for a proper Eyefinity setup. However, depending on what you believe, it is possible to use an active DP adapter to change DP into DVI. Now I've had nothing but bad luck with these in the past and I run a six monitor setup, two of which are DP monitors. I had no choice. I hear a lot of people got the active (read, ones that have a power supply of some kind.. USB or Wall Wart) DP adapters to work fine. You're mileage may vary.