Pros: Better water conditions than running your tank with out it.
Cons: having to check the skimmer cup every day or every other day.
Thats about as simple as it can get really.
For those who have a tendancy to overstock and overfeed a skimmer is going to help your water quality alot. If you stock your tank lightly and feed very sparingly then running with a skimmer would be less of an importance.