It is true that some leaves are very acidic. Walnut leaves are indeed very acidic.
So why do I munch them up anyway? Because I will only plant acid loving plants under them. Its no use fighting it. You can see the effect in Southern California where they have ice plants planted all over the highway hills, and they've filled in everywhere except under the evergreens where they refuse to grow at all. They are an alkaline loving plant. You can also tell when people try to fertilize their ice plants. Generally speaking the soil gets made less alkaline and they simply don't like it.
There are PLENTY of acid loving plants. I know blueberries are, and so are gardenias, rhododendrons, azaleas, camelias, hydrangeas, tomatoes, strawberries etc. Pretty much any kind of plant you find in a forest is acid loving.
Leaves don't burn that well - you won't cook a steak on one, and they really do make a decent mulch. The grass won't grow very well under those trees, and I will soon sow some sort of replacement cover. Some sort of strawberry cover might work well. I won't have to hassle with trying to make something grow where it was never intended to grow.
I do love a good grass turf though.
I could have just as easily hooked the bag onto the mower, and bagged them that way. They don't get quite as pulverized, but the mower vacuums them up quite efficiently.