One day when you're powershifting your clutch pedal won't stick to the floor when you overextend the clutch fingers, blowing your engine into a million pieces. Other than that not much. I run a McLeod 75209 in my Duster, no overcenter spring. Diaphragm clutches are not supposed to be used with overcenter springs.
Here's one of the times it's come up. There are a bunch of threads like this, just search for "overcenter spring". You don't have to take my word for it.