We recycle them for the same reason we recycle anything: to get more use out of it, so we don't need more oil to make new stuff.

do we want to dedicate to landfill?

with coworkers that would let spilled plastic pellets get washed away with the rain or ? Down the gutter it goes, to the sea. There, creatures ingest it, until it replaces much of their stomachs, so they starve slowly. FUCK THAT. Very much of the original energy to produce the plastic, is retrieved, by recycle.

In answer to #1 above, they will break down. Plastics oxidize and there are fungi that will metabolize them, but those are very slow processes.

In answer to #2 above, we shouldn't be dedicating ANY space to landfills. Everything can be recycled, even if you can't think of a use for it, you can turn the organic part into charcoal and use it as biochar.

Recycling plastic saves the energy needed to synthesize the plastics in the first place from petroleum. Consider the analogous manufacture of aluminum. Making aluminum from bauxite ore requires large amounts of energy in the form of electricity. Recycling aluminum just requires remelting it, a much less energy intensive process.

The only reason humans exist is to provide the planet with plastics, which it cannot provide for itself. Recycling slows down overall production of plastics and extends our time here. Once critical plastic mass is achieved, we will be eliminated.