I started doing this, maybe 15 years ago, but if I look through my spam folder now, most of it is to the email address I used before I began using unique addresses (the rest is to random addresses in my domains that I've never used).

My hypotheses from that are that

• there is probably less 'selling of email lists' going on than we think
• I'm less interested in dubious internet sites than I used to be
• or (most likely) these days, your internet thing has to be offering me some real value if I'm going to consciously give you any of my data.