I suppose I should have clarified. Of course, in wet conditions, the addition of fenders will not keep your bike dry. However, muck water coming UP from the road is of far greater concern to me (and usually greater quantities too) than precipitation falling DOWN on the road and my bike.
Look at the underside of your fenders after a wet commute, and see the dirt/crap/etc. That would have wound up on your downtube, kicked back into your chain, flung up into your headset, and yes...splattered all over you.
Of course, that's not 100% of the crap you ran through...but it's a pretty good amount, and I think that can save a lot of wear on your bike.
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Good night...and good luck