I use to do a lot of second runnings beers when I first started brewing. Then, after a while I realized that most of the 2nd running beers I brewed simply didn't come out well. Either they were lacking in flavor, thin bodied, or had astringency issues from over-sparging. Sometimes when completing the double brew day I would fuck things up and forget to add kettle finings or hop additions when both brews were in full swing.
Now when I brew, I focus solely on that one batch and make sure that I brew it to the best of my abilities on that day. I still dump batches that are not up to my expectations or if I get bored of drinking them. Hey, its only beer.....