Christ the generalizations in this thread are almost unbearable. There are guys who race fast that don't run anywhere near 6:00 pace or whatever on their easy runs. And there are guys out there hammering pretty hard almost every day. The generalizations though are just ridiculous. Oh yea 7:15 pace is more than 70% of your goal pace, might as well just pack it in, can't get any aerobic benefit from it. Jesus do you really believe that?
Where do you come up with 70% of your goal pace (also that term doesn't make a whole lot of sense, do you mean 70% less than goal pace?)
To the original poster I think the answer again depends. There are guys who can squeeze in more quality on easy days without having workouts suffer and there are people who for the most part need to keep these runs easy. Some guys have trouble recovering, some guys do harder workouts, and some are just injury prone. I think that most high quality college programs out there are of the type where guys are getting in some pretty high quality runs (read: not slow jogging) on non-workout days. But that's not to say slower running is useless. Also i think a lot of these programs where quality on non-workout days is stressed take even a couple of years for guys to settle in. Some guys adjust but I'm sure some don't. Eventually i'm sure one becomes more used to running faster on easy days but again to call slower running useless is dumb.
also to the guys saying 15:30 guys can/should be doing 6 minute miles on their easy days, what should the 14:30 guys be doing for easy days. 5:35's? Get real. 5:20's for the 14 minute guys? I just don't see it. I'm all for occasionally running a little harder when feeling good, losing the watch and running by feel, but 6 minute pace for a guy who runs 5 minute pace in a 5k is just dumb unless you're doing it as a moderate workout.
"With all that being said, I would think anyone who can run 15:00 - 15:30 should be able to handle sub 6:30 pace without any strain."
That's simply not true...some people don't recover at 6:30 pace. You're obviously very talented and some people can recover at 6:30 pace really easily (in your case sounds like it was never a problem) but most people can't. Also there's a difference between "handling" and "recovering at" a certain pace.
I also laughed pretty hard at the 14:20 guy who closes easy runs at 4:45 pace.