I don't consider myself a peak bagger, in fact I've made fun of them, calling them "list checkers" But I confess that I'm not immune to the disease. Early in my hiking career, I went on a couple peak bagging trips with the ultimate peak bagger, Bob Martin. Bob Martin was the second person to climb all the peaks in Colorado over thirteen thousand feet, and there are a lot of them. but he didn't stop there, he climbed all the peaks over 12,000 feet, but he didn't stop there. The number of peaks he's climbed numbers in the thousands. He's done a lot of passes too, even writing a book about them. He's also climbed most of the peaks in Arizona.
Bob Martin has heard the list checker criticism and his response was a good one. He has seen many, many amazing and beautiful things that he wouldn't have seen if he weren't trying to check peaks off a list (not his exact words, but along those lines). That's the best defense of peak bagging I've heard.
By weird coincidence, a long time friend of mine, who I've done may adventures with, became one of Bob Martin's last hiking partners,