Warning: I ran housing cables through the fork leg for 2 seasons before a retired builder set me straight
I figured it kept the lines cleaner than zip ties along the down tube, but a guy with 30 years on the bench at a bike co-op in Tucson pointed out the cable was rubbing a groove into the alloy right where it flexes. Pulled the fork and the groove was deep enough to catch a fingernail, so I swapped to a bolt-on guide that costs about $9 and now I check that spot whenever a wheel's off. Anybody else find a rub spot they'd been ignoring for way too long?