Hmm. I tend to forget that I know material. I've learned perhaps 50+ covers all the way through in the past couple years, but as noted, if I'm not playing them every week, they slip into long term storage, and I have to strain a little to get them back into my fingers. The occasional phrase is often easier to remember than the whole piece.
Bits and pieces of these tend to creep into my work on originals. I've found that learning covers is really great for expanding vocabulary - so I spend a lot of time just learning random pieces, recommendations I've found on here, and tunes that interest me for whatever reasons.
I try to make learning new material a regular part of my practice routine. I'm of the opinion that learning cover material is, at the very least, on a par in terms of importance with site reading, scales, technique, theory, and other practice elements. When studying english, we spend a lot of time learning how to spell and how to build a proper sentence, learning proper punctuation, and all of that, but, at least when I was in school, we also spent a lot of time reading and breaking down other written works.
To me, learning complete cover songs is comparable to this. We need to learn the theory, and we need to learn the physical movements associated with playing, but of at least equal importance, we need to learn how to apply that theory - we need to see WHY all this theory works.
/soapbox
