For scale purposes imagine a football filling most of your frame,, try getting that all in focus and sharp as possible. Thats why i like lots of dof.
Is using the 50mm prime lens on a 5D2 a good idea for this scale, as DOF will be greater that say the 100mm macro
I'll probably get this massively wrong and I'm sure the lens experts here will correct me, but I'll take a stab at it anyway.
In your example, I believe a lens with longer focal length could actually be better. Here is the way I understand it: there is a relationship between depth of field and the distance of the subject to the lens. The further you are from your subject the greater the depth of field in actual distance. To give you a gross example: when you are shooting with the lens on infinity, subjects that may be hundreds of feet apart will be in focus. On the other hand, if you are just a few inches from the subject, the depth of field is going to be very narrow.
Using your football example, a sort focal length lens that is close to the football may mean that only the stitching is in focus even with the lens stopped down because the relative distance between the front and back ends of the football are much greater. With a longer lens, (and presuming you have enough room to back up and still get the image framed) the relative distance between the front end of the football and the back end of the football is much less. So, it may be possible that by stopping the longer lens down you can get the whole subject in focus.
To use another example, many photographers prefer a longer lens for portraits because the relative distance between, say, the subject's nose and ears is effectively compressed by the longer lens, making it easier to get the entire face in focus. With a wider lens, that relative distance increases. (There is, of course, another major advantage to the longer lens, because it flattens the facial features, making the nose less prominent, whereas the shorter lens places the nose in much closer relative proximity to the lens than the eyes, making the nose appear bigger. Since most people think small noses are more appealing, the longer lenses are preferred).
Now, as I understand it, if the camera were to remain fixed, and you switched lenses, the shorter lens would have the same depth of field as the longer lens. But, since you are unlikely to be doing that, getting further away from your subject increases the depth of field. (As I understand it, this is why a crop sensor has greater apparent depth of field, because the actual distance to the subject is greater than with a full frame camera).
Okay, that's an explanation from a guy who never took a physics course in his life. I'll let others give you a more precise or corrected explanation. (Or perhaps explain why I am completely wrong)