Interceptions are not a very good way to measure quarterbacks. They're inconsistent from year to year, they're individual events highly influenced by factors outside the quarterback's control (can the defensive back catch? was the ball tipped by your own receiver? did the receiver run the right route? was he hit while throwing?), and they're only a small part of a quarterback's game.
Still, Football Outsiders managed to improve the statistic a little bit by factoring in dropped interceptions, tipped interceptions and interceptions that are 'acceptable' (on Hail Mary plays, desperation plays on fourth down and at the end of a game). They call this metric Adjusted Interceptions, and it's proven to be a lot more consistent for quarterbacks from one year to the next, signalling that it captures some area of skill better.
Note: I've charted games for Football Outsiders, so that may make a bit biased.
For Mike Glennon, very little changes with these adjustments. His interception percentage goes up a little to 2.6%, but that's a perfectly acceptable (and average) number. He had three dropped interceptions, which if I recall correctly all came in his first couple of games, and he had one interception in a situation when it didn't matter.
This isn't new. Glennon's deficiencies don't involve risky throw, but an unwillingness to pull the trigger early. If you don't throw the ball down the field, you're not going to throw a lot of interceptions. Glennon looks like he's improved a little this offseason, though. And it's good to see that at least his interception percentage is reasonably reflective of how he performed last year.
Josh McCown is still at the bottom of the list in adjusted interception percentage, throwing just three picks (two of them being dropped). That doesn't mean it's exactly sustainable, and part of the reason is that McCown played a very easy schedule. Another part of it is simply that throwing very few exceptions is more luck than skill, overall. It's just not sustainable to throw fewer than one interception every 30 passes over the long term.
Overall, the Bucs have two quarterbacks who, at least in the past, have not thrown many interceptions. And that's very, very important to NFL coaches but vitally important to defensive coaches like Lovie Smith. It's the first thing they look at when evaluating quarterbacks: will this player not put my defense in a bad position? Which explains why the Bucs now have two QBs who are at least nominally good at that one specific aspect of quarterback play.