Obviously you can't "put a spike on pretty much anything" and have it be effective, or it'd have been a much more common practice. Spikes were not added to Viking axes, or Indian maces, or English hammers, or Eastern European flails. Spikes do appear on certain polearms, but at that point you're already starting with an unwieldy weapon and there's little penalty to adding yet another projection to the head.
I've actually toyed with the idea of breaking out melee weapon damage into different types, and changing the rPD of armor accordingly. In that case I'd have taken away the +1 from swords, but given them the ability to choose between (effective) piercing or slashing damage. Never got around to fleshing it out though.