The rationale is that he's the top player in the draft. Best available player. Until today, the guy was ticketed to go 1st overall. And you're getting that quality to replace Doug Free?!?!? It's not a move based on dire need, but those seldom work out anyway. It's to add to a strength, making it unquestionably dominant, and giving Romo or whatever future quarterback the greatest chance for success.
I think that many, including the team, would disagree with your assessment. Despite everyone's hopes and claims last year, the team put him at left guard and kept him there. While people continued to talk about a right tackle that never happened.
And for the record, I think they're hanging on to Leary as a backup for injury for a year or trade bait. But I see no plan that involves putting him back on the field to start.