Agreed, but thats less than a tenth of the cost of the lens. Only problem is a lot of amatuers have £200-£500 lenses so you've got to weigh up the pros and cons. I recently bought a 50m f/2.8 and wondered what to do. A tenth of that would be £24 so I dont know if it would be worth it. There's the Sigma EX DG UV or Hoya SHMC Pro-1 Protector for that kind of money which I havent 100% ruled out yet.

Would you consider buying a filter (at whatever price you consider approriate) and take a couple of shots without it, then fit it and take those shots again. If you're happy with the results, keep it on, if not you can always take it off for shots that matter but leave it on for "site survey" shots or shots in the rain, etc?