I don't think it's that big a deal at all storing ground grain for weeks or months. I normally grind my own grain, but have many times used pre-ground. I think the biggest enemy of grain, whether pre-ground or not, is humidity. If it is kept cool and dry, I really wouldn't worry. If your grain is ground 5 minutes, 5 hours or 5 days before you mash, I'd bet a lot that you'd get no difference in yield of efficiency or flavor or anything.
I think the only real advantage is economy, and that unmilled grain is slightly less susceptible to humidity and the resulting breakdown. Assuming you have appropriate storage, pre-ground or whole grain is really good for quite a long time. I keep a homebrew store's worth of grain on hand at most times so I have flexibility in recipe formulation. It stays good for months and months and months in sealed tupperware-type containers.
Remember not to get lost in the details. tnlandsailor may only use grain ground within 5 minutes of mashing, but he doesn't use a secondary fermenter, so...you be the judge
(just joshing you, Dennis...I don't even use a hydrometer after all
Bottom line for me is that owning a grain mill is just kinda cool and makes the whole process more manly, because now you have to use a power drill and a mill and all the noise and dust... WOOHOO!!
However, I have bought 50 pound sacks of pre-ground grain (ground at a brewery, not a brew store), and the grind was better than anything I could do with my fairly nice homebrew mill. The big boys have much nicer mills than you'd ever want to buy, so some of the pre-packaged pre-milled grain is a much better grind than any homebrewer could reasonably achieve. So, to me, the "quality of grind" argument doesn't hold too much water.
Just think noise and dust and power drills and 50 pound sacks of grain...you'll buy a mill in no time