Another example about activity, I just started editing a category, that did not have an editor. So there's a pile of sites there waiting to be reviewed. But my main reason for editing that category, is to move some web sites out of it, to a new category that better fits them. When I've finished doing that, I may stop being an editor there - since the rest of the sites are not in my personal interest, leaving a lot of sites untouched. There is only so much much a person can do.
But someone already noticed that I'm now an editor there, and emailed my complaining that she'd put her site in two weeks ago [patience, patience, I've got one of my own sites been waiting for three months], and how come it wasn't in yet.
Now if her site is included in the ones I'm interested in, it will get done in the next week or so, if not, maybe I'll never touch it. So the acivity date, may lead you to false conclusions.
P.S. I have another category where someone complained about his site not being in, turned out I'd only been editor in the category for about 4 hours, and had already put it in.
So from a certain point of view, it would be better if ODP did not show the edit date or the editor name.