As Kevin mentioned, the semi-apo is a combination of the Fringe-Killer and the Moon&Skyglow filters. See image:
http://stargazerslounge.com/uploads/...1379628240.jpg
I think this was done to try and correct the yellow colour imbalance that the Fringe-Killer exhibits. A handy benefit of this is that it would help reduce the effect of light pollution from Sodium Vapour streetlights.
I can't comment from experience, however I did research these filters extensively to be used with my StellarVue SV70 doublet which exhibits severe purple/blue fringing.
In the end I decided I would have to buy a number of the filters to find one that worked to my taste and that would have cost more than the scope!
I then decided the best option was to wait for a better corrected doublet (or even splurge on a fluorite doublet or a good triplet) to pop up in the IceTrade section, but I've always just missed out on those that do come up for sale. Gotta be quick around here!