Let's take those war criminals (of which dozens have been tried and sentenced) from the Balkan conflict as an example. Here is a group that still has lots of support (Serbs primarily) all across Europe. They are in cells in the Hague which is driving distance from their homeland. Not like some poor Afghan farmer totally divorced from his people, these people have strong support living with a few hours drive!! Almost nothing could stop them from attacking and trying to release there leaders (and heros), or at least taking revenge on the country they are incarcerated in. The REAL danger to this court pails to anything the perceived Gitmo people could possibly do.
Just look at the history of the Balkan conflict, its horrible genocide and the people who did the killing, and then grab a map to see where the two countries lie, you will get the picture. Then do the same for the Afghan conflict ... Kinda makes you giggle.