Counterspell is already a spell, it is called dispel magic. I assume there are spells that are available to everyone no matter where they've chosen to put their books and if so this would be one such spell. I don't think that replaces the ability to counter a spell by identifying it and casting the same (or a special counter noted in the description). Using only dispel magic to counterspell significantly reduces the value of the spellcraft skill, prohibits lower level units from counterspelling at all, and almost forces everyone to have to research dispel magic.