The cost of the skimming unit is not worth it in a fresh water tank.
The marine water in Marine Aquariums is quite expensive. Because of the high cost of marine water (RO unit and running cost, salt additives etc), it is cost effective to use a Protein Skimmer to remove waste to limit water changes. 100 gallons can cost about £5-10 quid.
How ever a freshwater aquarium only needs tap water and a little dechlorinater which is a few pence a gallon if that. So you can just do water changes on fresh water to remove waste.
Yep, what Betta5 said. The physics of FW means that a protein skimmer won't do anything in freshwater. If you just have a FW setup, it will be nothing but a waste of money.
There are protein skimmers that work in Freshwater but otherwise I agree they seem to be pretty worthless. Unless you have a lot of carnivores or something anyways but then I'd be doing more water changes to counter all the waste.