Keeping lines clean will definitely help, besides that they sure do cast nice when clean. I think many of us, me included, would enjoy casting much more if we cleaned our lines after every outing. The benefit of cleaning doesn't seem to last long for me though, maybe I fish dirty water?
It appears as though I have stumbled upon the answer. PVC lines (all but the AirFlo product which is PU) contain plasticizers to keep them subtle and crack free. Over time and with use in the sun (in the sun...go figure!) PVC will leech plasticizers causing them to stiffen, crack and essentially "turn to custard"--Sadly, I have some of my favorite spey lines that have done just this as well as some of my trout lines. These lines need to be re-plasticized with a line cleaners that does more than just get the crud and dirt off but also infuses plasticizers back into the fly line. From what I can tell most all line cleaners do not use plasticizers (according to their labels).....all but one that is. INTERESTING!
PS- I called a local fly shop about this today.....they were clueless (or at least pretended to be after "having sold lines for 17 years").