What is WITH this obsession with the 3-4? If the 3-4 was the magical cure-all panacea BESTEST DEFENSEIVE SCHEMEAGE EVAR that everyone seems to think it is, wouldn't everyone in college football be running a 3-4 by now?
The reason it's not used more in college football is because there aren't many guys at this level that have experience running it and no one at any major school is going to just start experimenting with it.
The only teams in the NFL that use the 3-4 defense are the Steelers, Cowboys, Patriots and the teams coached by Parcells and Belichik disciples, and I think Weis is only the second in this coaching tree to be a HC at the college level.
As more NFL coaches come down to college football, the more they will bring guys with the knowledge of using the 3-4 defense, who in turn will teach others and the teams using it will grow.