I don't understand what you mean, You said ATI cards only have 2 clocks per GPU, so they could have actually done this with a 5970 instead of using a DP. I thought that could not be done. I see the point with Eyefinity 6 cards using all DP but not on the 5970.
Yes, they could have done this with 5970. Why they didn't, I don't know, but its not because of lack of clocks to drive 3 DVI's at least. If to speculate, it might have something to do with the inhertied issues of running 3X screens while syncing it over 2 GPU's and therefore, they chose to use a single GPU for displays instead.
With issues, you can find some here (bottom half):
http://www.anandtech.com/show/2937/10
There is also matters of power and heat in 2D mode, since there are issues with cards having high power usage on second card in 3X setup with Nvidia. Ati might have looked to avoid this by running all 3 screens off one GPU.