Yes, they are usable, at least if they have reached a minimum degree of “ripeness”.
There are a few reasons of falling fruit. The first is the so-called June drop, when the tree discards excess baby apples. These fruit are so tiny and unripe, they don’t have real value in the kitchen.
During summer, fallen fruit is closer to ripeness, especially if infected by codling moth caterpillars or in early fall, when wind causes almost-ripe fruit to drop. Just try a slice and decide whether they have enough flavor already.
These fruit can be used in the kitchen, but you need to consider a few points:
- The fruit will be less sweet than usual - can be fixed with a spoonful or two of sugar. Likewise the flavor may be less complex than usual, adding a bit of spice will help hiding this.
- You will have difficulties cutting neat slices. The apples will be bruised where they hit the ground and if you have moths, there will be a “channel” full of gunk that needs to be cut out.
- The cooking behavior (keeping the shape vs. falling apart) may be different.
- Higher pectin levels in underripe fruit can cause unexpected thickening.
Because you will be cutting off a comparatively large part of the apples, I would collect mainly the largest, ripest (scent and color can be an indicator) and least bruised. This should still give you a decent amount of fruit without too much work.
And you are not alone: