It's widely accepted that teams that build consistently through the draft are more successful. Continuity matters. The Falcons have stated, and have demonstrated, a commitment to retaining the players they draft and develop.
As a matter of fact, this interactive graphic from NFL.com, featuring data from 2007-2011, illustrates the connections between teams' win percentages and how many free agent acquisitions they had on their roster. In general, the teams that seemed to focus more on building via free agency had lower win percentages.
In the Dimitroff era, from 2008-2012, the Falcons have drafted 38 players, and have retained 26, or 68.42%. The players they have retained have started in 465 games, including postseason, which breaks down to 17.88 on average. That tells us that the player they are retaining are starting-caliber players. The players they drafted but let walk during that timeframe collectively started in 97 games for the Falcons, for an average of just over 8 games each.
Keep in mind that the drafted players that the Falcons have retained include guys like Bradie Ewing, who spent his rookie year on injured reserve; Matt Bosher, who doesn't get starts attributed to him, and Jacquizz Rodgers, who isn't typically a starter, but has certainly been an impact player.
Some people do argue that the players that the Falcons did not retain, generally being below the standard for a starting-caliber player, creates a case for poor talent evaluation, because they drafted players that weren't very good and then had to part ways with them, but considering that the Falcons have retained such a significant number of players, and that those players have started in a significant number of games, thus directly impacting the team's success, there's plenty of evidence of Thomas Dimitroff's skill in evaluating talent.
This information is based on some really good data from drafttek.com. This breakdown for draft pick retention for teams that made the playoffs last year is based on the 2009-2012 seasons.
All of the teams that made the playoffs last season have retained a significant number of their drafted players, and the Falcons are near the top of the list, just behind the current champions, the Baltimore Ravens.
Out of the 58 players currently on Atlanta's roster, 31 of them are draft picks Falcons have retained (including draft years previous to Dimitroff era.) There are two asterisks: on the Falcons list, cornerback Dominique Franks and linebacker Robert James were both excluded, for a total of 29 drafted players retained. Franks was likely excluded because he was cut prior to the 2012 season, and later re-signed with the team.
There's no surefire formula to building a competitive football team. Draft picks are always a risk. A guy can get hurt in his first day of camp, or he can turn out to be not built for the rigors of the NFL, or he can, quite frankly, just inexplicably suck. And, you can't just rely on the draft as a GM--you have to be willing to bring in veteran free agent talent as well, as the Falcons have done. But the Falcons have, in the Dimitroff era, drafted in such a way to build a competitive team in perpetuity.