Some fans were debating in the run up to the cut down whether the Colts would keep three QBs or two, like they did through most of the Manning era. The Colts have the best #2 QB that we have seen in years in Drew Stanton, but Chandler Harnish look very good as well. Harnish looked better than a number of #2 QBs we've seen this preseason. Keeping three QBs became a no-brainer once Chandler showed how well he can play. For one thing, the Colts only have Stanton under contract for 1 more year, whereas they have Harnish for three. But the most important reason to keep three, is that many teams around the NFL have backups that suck.
Once the #1 QBs start getting injured around the league, Stanton could be a hot commodity. Harnish's quick development allows the Colts the luxury of trading Stanton, should the opportunity arise. The Colts got Stanton for a 6th (or 7th - not completely sure) round pick - given the right situation they might get a conditional 4th or 5th rounder from a team in need. Grigson has been doing a great job of building depth in this organization - there is a limit to how many starters any team can acquire in one off-season, but greatly improving the #2s and #3s on your roster can have a big impact over time. Some people have been upset at the draft choices that have been traded away, but improving the depth of the team should start paying off in draft choices coming back this way at some point in the future.