It depends on many factors:
- How often would you ride on wet pavement or gravel? I really like it when it looks like I won't get a ride in because of the rain, then it stops and turns beautiful. Ground is soaked, air is moist, fragrant and non-chapping. Without fenders, I still might not go. With fenders, it's a no-brainer. I had a ride like that this past Friday night, after it had rained all day. I took my fender bike; no problem.
- If you have multiple bikes, one of them should have full fenders, assuming you have a mixed climate.
- Where do you live? If it rarely rains, maybe it's not worth the bother.
I have a few bikes with full fenders, a few bikes without fenders and two bikes with clip-on fenders like John Flores shows. Those clip-on fenders are a good option for gravel bikes:
This gravel bike has a front fender strapped on the downtube. It's nice to keep the water spray out of my face. The Topeak BeamRack acts as a rear fender. There's plenty of clearance for rocks and a bit of mud. Before I got the BeamRack, I had a rear fender that clipped on the seat post.