I read on viewfromthewing.com (mirror):
To be sure, your boarding pass still has a bar code. Someone with a bar code scanner or app could read it. But that’s not the person most likely to pick up your boarding pass. It actually seems more likely that someone would pick up your boarding pass and add your reservation to their Business ExtrAA account [sic] for small business program credit (in addition to you receiving your miles).
Are there any downsides for me if someone picks up my boarding pass and adds my reservation to their Business ExtrA account for small business program credit?