15 interceptions is not a good year. I'm not sure it makes the perfect cut off point, but it might be as good as any. I can't off the top of my head think of any modern era seasons in which a QB has thrown MORE than 15 picks and had what we'd consider to be a good season.
I don't know what one would consider the modern era. Wikipedia tells me the 70s and 80s, which doesn't seem modern at all. I'd argue the past 8 or 9 years have been a completely different era after Peyton and the Colts got a bunch of rule changes implemented. I've already pointed out two NFL MVPs who have thrown more than 15 INTs (Manning in 2009, Warner in 2001). But even if you just became an NFL fan this season, you would find 3 examples in 2014 alone.
Andrew Luck (38 TDs, 16 INTs, 95.4 RAT) and Philip Rivers (31 TDs, 16 INTs, 95.8 RAT) have both thrown more than 15 INTs with 1 game to play and both have been mentioned in the MVP discussion this year. They've dropped since that time, but I have no problem saying they've had a good season. And Manning's 39 TDs, 15 INTs, 102.9 RAT are pretty damn good this season too. If the season finished today, it would be the 5th-best season in terms of RAT for Manning.
Drew Brees co-led the league in INTs (19) in 2012, but he also led the league in TDs (43) as well as yards (5,177). In 2010, he threw 33 TDs, 22 INTs, and 4,620 yards. In fact, Brees has 7 seasons of 15+ INTs as well. They're not all the same of course. Early years in San Diego like 2002 (17 TDs, 16 INTs, 76.9 RAT) and 2001 (11 TDs, 15 INTs, 67.5 RAT) are not the same as 2012 (43 TDs, 19 INTs, 96.3 RAT) and 2008 (34 TDs, 17 INTs, 96.2 RAT). We can at least agree on that right?
We covered Manning's 2009 MVP season but Matt Schaub also had one of his best seasons with 29 TDs, 15 INTs, 4,770 yards and 98.6 RAT. In 2007, Tony Romo led the Cowboys to a 13-3 record while throwing 36 TDs, 19 INTs, 4,211 yards, and 97.4 RAT. Romo also had a second season in 2012 where he tied with Brees for the INT lead, throwing 29 TDs, 19 INTs, 4,903 yards and 90.5 RAT.
These are the great seasons. I'm not even going to start with all of the "good" seasons with 15 INTs or more because there will be lots of bickering whether Matt Ryan's 2013 26 TDs/17 INTs/89.6 RAT should count or Andy Dalton's 2013 season with 33 TDs, 20 INTs, 4,293 yards and 88.8 RAT which we would accept in a heartbeat from Jimmy G or any non-Brady Patriots QB.
15 INTs is usually a rough season, I agree. But it certainly doesn't tell the whole story, or even come close. When it treats Manning's rookie season of 26 TDs/28 INTs/71.2 RAT season the same as Manning's 2009 MVP season of 33 TDs/16 INTs/99.9 RAT season, it's obviously a terrible way of rating QBs.