Sure. What about code written by others? What about when you're debugging and something just isn't working the way you expect because you've unknowingly stepped outside that subset? (Most of the important concepts here aren't syntactic constructions you can avoid trivially by typing Y instead of X, and may or may not be detectable statically). Also, maybe you're missing opportunities to better insure your code is correct at compile time - I'm not sure whether it falls within your subset, but for instance I recently put together a macro that asserts statically that two expressions have the same type (and without any runtime cost).

"I've also seen some C 'experts' fail to come to a proper solution even if they know all the features."

Obviously. Knowledge of the spec and your compiler aren't the only thing that matter, by a long shot. I'm just saying they most emphatically are helpful.