While I agree that poor penmanship isn't eye appealing for autograph collectors, I also try and think of the athlete.
Think of how many stickers they are signing for these companies - that's a lot of writing. If you have inserts by themselves, with serial numbers of 250, 125, 75, 50, 25, 10, 5, and 1, quick math shows that being 541 signatures. That's for one set by one manufacturer - throw in Heritage and Archives as well, for example, and using the same SNs, there's 1,623 signatures. At some point, I'm sure my penmanship would also deteriorate doing that.
I think there's also something to be said for the second go around, when a second-year athlete realizes what the heck happened last year, so they don't even try this year, it's just a scribble.
I'll never quit collecting entirely, but I am downsizing. Check out my COMC store and help me thin out what I don't want so I can buy cards that I do want. See something you like? Send me a message on here, and we can knock the price down quite a bit. I'll even take a bit of a loss if it means getting you a card you really want.