Why CO2 once over 3 watts/gallon?

Depends on how you define harm. the plants will grow slower, but if there are enough nutrients left over, that additional light and nutrients will encourage the growth of 'non-plant' things, like algaes and bacteria (the nutrients, anyway, bacteria don't care about the light). So, the plants aren't harmed directly by being CO2 limited, but the overall balance of the tank will be out of whack.
 
Black Brush Algae

Black Brush Algae, black short bristly algae, very hard to scrape off, no fish eat it. Very ugly. Typically worse in tanks with low CO2 in relation to lighting.
 
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