What in the Nine Hells is the point of half-plate armor?
Starter armor for low-level characters who can't yet afford full plate. Or armor that looks different so the GM can have his monsters look different.
I've always found it funny how many players try to buy full plate at 1st level before realizing that they can't. Even if I start you off at 2nd level, a suit of full plate leaves you almost no money for anything else.
Perhaps its just me that prefers taking scale mail or banded mail over half-plate. I'll go with what James said at the end there.