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There are 2 1/2 major cons to not having a hood/top:
1) Fish can jump out of the tank.
1A) Pets can jump into the tank.
2) The evaporation rate will go up so unless you are using distilled water to refill the tank, your hard chemicals levels in the water will be rising with every addition.
The pros for this is basically allowing plants to grow taller and the lighting wavelength will be brighter into the tank. ( The difference amount will be negligible depending on what material the hood is made of. )
One other option to address the cons is to use eggcrate. That can stop pets from getting into the tank and depending on the species you have, may stop the fish from jumping out. The downside is appearance. The eggcrate will be seen above the tank.