The mistake is putting every run that isn't a "workout" into the same LSD/maintenance bucket. There's room in a training schedule for a range of paces slower than MP.
If you're going 3 minutes/mile slower than MP, that's not "maintaining" anything. That's basically a recovery run. After a difficult workout, your blood lactate levels can remain elevated for quite a long time, and research has shown that easy exercise will bring the level down to baseline sooner. Note that this isn't "flushing the lactic acid out of the legs," as some coaches say. Lactate clears the blood quickly. If your levels are elevated it's because you're continually producing more of it. Another benefit of recovery run include a spike in anabolic hormones. It's also likely that recovery running allows your muscles to heal from hard workouts while maintaining a wider range of motion. It's also possible that by running again before you're fully recovered, you are stimulating your body to continue repairing itself as quickly as possible. Keep in mind, also, that recovery running doesn't have to be this slow; it just has to feel subjectively very easy. Recovery running could be 90 seconds slower than MP, if it's not too long.
If you want to get an aerobic stimulus from your non-workout days, you should be running much faster than your recovery pace. Within around 40 seconds/mile of your MP. That's the very outer edge of when blood lactate levels will creep above baseline and muscle oxygen will start to dip (actually if you're in true PR marathon shape, you'd have to go faster still, but if you're in the middle of training and a ways from your goal race, 40 seconds is fine). That's what you need to stimulate adaptations. That's pretty fast, so you likely won't be able to do that for all of your runs. You'll be too beat up. Try it for 1 or 2 runs per week, or try mixing in those paces as a brief progression at the end of easier runs (just 10-15 minutes).
There are also non-aerobic, non-recovery runs. These are often what people would call a standard distance run. There isn't much aerobic benefit to these runs unless you stretch the distance. If you do make them long (whatever long means for you at the moment--for beginners it's a couple miles), then you can gradually exhaust the aerobically trained muscle fibers and start working on the ones that are not so well trained. You'll typically realize that's happening because maintaining the same pace will start to cause an increase in respiration. If you're not pushing the distance particularly far, then I think most of the benefit here is in developing fatigue resistance in the working muscles themselves. This is a huge part of running--especially marathoning--and a lot of the leading theories don't focus on this. They mainly treat running as dependent primarily on energy systems and oxygen transport. Another likely benefit of this kind of middle-pace running is keeping the weight off. Skinnier is faster in running.