There are a couple kinds of prefilter socks on the market, one just for heavy dust and one for moisture repellency. In reality, if you keep the "sock" clean it doesn't affect air flow enough to worry about on a street bike. If you have an exposed element filter and want it to look decent without having to soak it and re-oil it every week, or wash and dry it every week in the case of the H-D paper element, a prefilter sock is a good idea even when it isn't raining.
I think using a prefilter all the time like Highjagger does is a good idea, just make sure you clean the sock often. And it wouldn't be a bad idea to have a spare on hand. I like the idea of the different colors sold by folks like K&N so you can change colors to suit your mood. I haven't measured the air cleaner on a SERG, but you might find one that fits on the K&N site, possibly in a choice of colors.
http://www.knfilters.com/search/wrap.aspxBTW, K&N indicates that the water repellency will diminish over time. I'd imagine that will also be true with the H-D version, so you might want to include them on your normal maintenance checklist as a yearly check. Spritz a little water on it, if it doesn't roll right off get a new one.
Jerry