Bob,
What's the percentage, 90% of the intake water is discharged as brine I think? I am assuming this process is what keeps debris from building up. My guess is that the back-flush option on some units is more about protecting the membrane from bacteria. Thus with a Katadyn, the trick is to either use it every few days or preserve (pickle) it. Pickling isn't very hard to do, so that's the state mine is in right now.
Here's something the Spectra rep also told me that I haven't been able to buy into yet. He said to feel free to use the heck out of the unit, in harbors, creeks, etc. Sure, oil will certainly ruin a membrane, but oil is on top of the water, not down by the raw water intake (except for some areas of CA where oil can come up from the sea floor). He says they've got customers running Spectras in Baltimore harbor without issue. I always thought they should only be run in "blue" water. I did run mine in the muddy creek my slip is in when I first installed it and it certainly output good, clean water, but I'm too chicken to trust that I wouldn't ruin the membrane if I did that all the time.