What's the harm in trying to build for the future?
As of 2012:
Since 1999
442 total players have been drafted in the 3rd round.
55 have started 3 seasons of more.
29 have started 5 seasons of more.
10 have started 7 seasons or more.
The odds of any pick working out isn't great. Why not take a high upside QB who could potentially replace a franchise QB or flip for a pick? If it doesn't work out, you really aren't missing out on anything. 29 players starting 5 seasons or more in 14 years? That's not even one 3rd rounder per team. In 14 years.
It was them thinking they could flip an unknown QB for a better pick in the future when they took him, nothing more. People need to forget about the Schaub and Kolb situations, because teams don't give up high draft picks for unknown back up QBs.
If you aren't getting a starter in the 3rd round anyways, why not try to get a higher pick out of flipping the player? It's a high reward low risk chance.