We're talking about small amounts here, that aren't replicating. If you have the virus coursing around you get a lot of contact with the real spike protein, which can replicate inside your body.
The body is used to fighting off a lot of viruses and bacteria at the same time. Why would this spike protein, without the attached virus, be different?
After all we can't compare the vaccine to a world without the virus, since we won't get that back. Instead we need to compare the vaccine to the world with the virus.
Maybe it does not matter much, but the immune reaction can indeed be very strong. Just don't be completely naive when in comes to regular booster shots, there might be a fine line between benefits and negatives. Also other types of vaccines are on the way I heard, so we'll see, maybe that will open up alternative ways to combat the virus.
I don't know, maybe its all fine and good, but if you ask me, I think a lot of people are way too excited to be part of this study. Especially since the risk/reward doesn't seems to be that great in the first place.