For professional astronomers, it really sucks if you only have, say, an hour of observing time on a big telescope, and then a few of your precious frames are ruined because of a satellite. And you don't always even know that the frame is ruined until later, or have the flexibility to take more frames if needed (because you submit the full observation plan ahead of time, and just get the results later), in which case one bad frame can mean you don't have enough good frames to get valuable data.

Or more generally, let's say 10% of frames have satellite noise. That means there is 10% less observing you can do in a night. For amateurs, that is annoying. For professionals, that means less science can be done.