if you go away for a week then you want the overflow thing you can get.
Some ppl say when its breaking in the cup fills up real fast, some ppl say it starts slow but works better after a few weeks.
I have one and empty it about once a week and that's witout the overflow box.
I have one and don't have the overflow box though there have been a few cases when one would have been nice to have but that was caused by using additives the skimmer didn't agree with. Now my skimmer cup rarely fills up anymore. Though this could be because I have just recently gotten back into the hobby and maybe my water is just really clean right now.