are you reffering to a pre filter for your skimmer?
If so , wheras your skimmer is in so much goo, I might consider one, they are not essential, but it would probably help your kimmer, however, you might wind up having to clean your pre filter more often.
If not, and you are refferring to the skimmer as being a filter, then, they can be a great help, at keeping your nitrates down.
Do you have a reef, aor are you planning on going reef, in the future? If so, are you having any troouble with your nitrates? Wet/dry s have been known to contribute to nitrates, if you are having a problem, keeping them down, you may wish to consider turning it in to a sump or refugium, in the future(just remove the media slowly , alittle at a time; to avoid any spikes).IMO a refugium is better for nitrate control, and a good bio filter is better for filtration (dsb&lr), than a wet/dry, however this is just IMHO
HTH and good luck