People who think they need to save every bit of weight (what - 500g??) may delete it, but yes, it pollutes more as the unburnt hydrocarbon vapours have a worse global warming potential than CO2, and it will also mean your car will stink of petrol when it sits in the sun and the tank vapours expand and push out the end of the uncapped hose. If you cap the hose then it can lead to pressure build-up in the tank...not desirable!