Green Bay had a few years in the Mike Holmgren era and just before when they would take late round QBs, try to coach them up, and trade for higher draft picks. But that started in the era of the 12 round draft. They picked up Don Majkowski with a 10th round pick in 1987, another 10th rounder in 1988, a 3rd AND a 4th on forgettable names in 1989, a nobody in the 10th in 1990, then got Ty Detmer in the 9th round in 1992, Mark Brunell in the 5th in 1993, Matt Hasselbeck in the 6th in 1998 (with a couple of nobodies late in 1995, 1996, 1997), then that strategy seems to have ended around when they picked Aaron Brooks in the 4th round in 1999.
For their most successful years running this strategy -- 1992 through 1998, when they bought low and sold high on Brunell and Hasselbeck, -- they had Sherman Lewis as OC and Andy Reid on the offensive staff.
If Holmes believes that Ben Johnson is capable of coaching up a QB drafted in the late rounds then arbitraging him for a higher draft pick three years later, I'm OK with that, but even in GB that strategy doesn't appear to have been all that successful. Otherwise, I prefer them not to spend draft capital because "we need a backup QB." Instead, make sure you have solid backups on the OL so your running game and pass protection don't suffer if one of them gets dinged.