In layman terminology:
Scenario: If you're a boss and you have an employee that comes to work late/incompetent/give attitude problems, would you rather not have such an employee and do the work yourself?
Argument: But having more manpower is better than no manpower right?
You decide.
With inefficient skimmer (equivalent 5% WC) , your weekly WC is 10%.
Without skimmer, with frequent WC, your WC is 20%. This difference in 5% already overpowers the inefficient skimmer.
(this is just for your understanding. I'm not saying skimmer is comparable to WC)
Besides, skimmer also contributes to heat and takes up space. For smaller tanks, it is easier to do frequent WC (than larger tanks) which already does the work that a skimmer can do. For your goodbye po4, WC will remove the 'particles', you do not necessary need a skimmer for that.