Buck Turgidson wrote:
Partially out of laziness, inertia, procrastination, and partially because I
use it several times a month, I leave my Porter Cable air compressor full.
Does this do longterm harm to the machine?

Yes and no.

Typically, the smaller the compressor, the less efficient, and the
more condensate it generates.

Not emptying the tank and bleeding off the condensate on a frequent
basis leads to problems.

Lew