That's what I thought originally too. The water gets in, and it can get out easier too. If I ride with no shields, I run them totally stripped and dry, so stuff doesn't want to stick to them. Dirt can get in, but it can get out too. Really they just sound cool in footage when you can hear the wheels spinning in the air. Makes the footage more dynamic and appealing to the senses, yada yada, profit. Everybody does it. I'm not getting paid, or getting free bearings, so I'ma run em with shields from now on. I'm officially old I guess lol
I rode in the rain with my friend to get beers one time, no shields, and my swiss 6's rusted together later and wouldn't move. I revived them and they're still going today but that experience definitely helped me with deciding to keep the shields on and just oil them boys up now and again, which ended up working better than I expected.
That, and switching all my cup and cone bearings to sealed cartridge bearings on my bicycle because I'm a lazy ass and I also ride in the rain a lot. Got me thinking about how I wouldn't pop the caps on my bike's bearings... But like, track cyclists and time trial/crit dudes were known to run all their bearings shieldless to save weight, and using a thin oil instead of grease for less friction. This was in controlled environments though and not going on a beer run in the rain, but yeah. Marginal gains.
I also have been trying to simplify everything and not tinker with my stuff as much. I stopped messing with my trucks, and now I can also just open the package of bearings, put em in my wheels, and go. No extra steps