I have fenders on my commuter and distance bikes because it rains for 9 months out of the year. Commute 30 miles a day for near-record-breaking 27 day stretch of rain, or ride a 300k in the rain and they start to seem like a wise investment.

I have some crud blockers on my CX bike for training rides, but they're just clip on dealies that I can remove easily for race days.
I want to get a short front-guard for my enduro bike because it will keep down the amount of muck getting thrown up at my face on super muddy descents.