This Comic's Cast:

Okay, so this is really a Castlevania in-joke. Keys only show up in a select number of games, and in the later ones they're mostly used to unlock alternate paths. In and of themselves, in that function, they are very useful.

However, when you are carrying a key you can't use any other special item. This was used to obnoxious effect in Dracula X for the SNES where you had to carry one key for four damn levels just to unlock all the secrets you needed to and get the best ending (and, most galling, you didn't get Maria to play with for all that effort).

Plus, if you try to item crash a key, Richter just gets really confused and stands there. Keys serve no useful purpose if you aren't interested in secret areas. So, as Hanz says, they are entirely useless.
 
    

When evil spreads across the land, and darkness rises and the monsters roam. When the creatures of the night make beautiful music, and the things that go bump in the night go bump with greater enthusiasm. When the world is in peril and is in need of a hero...

These guys are, sadly, the best the world can hope for. These are the adventures of the heroes of CVRPG. They mean well, they try hard, and occasionally they do the impossible...

They actually do something heroic.