It keeps getting kicked around by writers, analysts, and fans. The same reasons emerge.
"Mike Tomlin has a 4-3 background."
"The Steelers are drafting 4-3-type players."
"The 4-3 is the 'in' system now."
Wrong, wrong, and wrong.
First of all, Tomlin is a Tampa 2 guy. If anything, the Steelers would adopt that system—which, judging by the team's secondary, would be something akin to professional suicide.
Secondly, the Steelers are not drafting 4-3-type players. They are drafting versatile defensive players who can be adapted to their needs in the 3-4. Ziggy Hood is not a 4-3 end or tackle necessarily as much as he is a good defensive lineman.
Finally, the 4-3 is not the "in" system. The 3-4 is. The Green Bay Packers adopted a 3-4 defense this year. Several other teams are doing it, considering it, or have done it. Part of the reason, believe it or not, is that the Steelers have had so much success operating the 3-4 defense.
Let me blow another hole in this argument.
The Steelers are not switching to a 4-3 anytime soon.
The team's linebackers are not suited to a 4-3 system. In that alignment, the effectiveness of James Harrison and LaMarr Woodley would be neutralized because the 4-3 does not utilize edge-rushing linebackers.
While Woodley could possibly switch to an end spot on the line, Harrison would not be able to make that switch.
Lawrence Timmons is not a prototypical middle linebacker either, so I'd be wary about the Steelers using him between Woodley and Harrison. He's another speed rusher who benefits from the three man line's hole-opening ability.
The line isn't even designed that way. In a 4-3, the defensive tackles are supposed to penetrate the backfield and wreak havoc. When is the last time you saw Casey Hampton go one-on-one, bust into the backfield, and wreck a quarterback?
He's the ideal 3-4 nose man. He two-gaps, takes on the center and one of the guards, and opens up holes for the linebackers.
Aaron Smith and Brett Keisel would operate effectively as 4-3 ends, but they aren't ideal as speed rushers either, so the whole line would have to be scrapped.
A 3-4 defense is built on blitzes and pressures, which take the heat off of the secondary.
Again, judging by their performance in 2009, I think the Steelers secondary is under enough pressure now. A 4-3 switch would also neutralize some of the things that Troy Polamalu—when healthy—does so well. His trademark blitz packages would be gone. A 4-3 defense, by nature, cannot blitz as often or in as many exotic ways as a 3-4.
Add to that the fact that Dick LeBeau, inventor of the zone blitz and soon-to-be Hall of Famer, is the team's coordinator for the foreseeable future, and you have yet another reason not to switch. LeBeau is a 3-4 disciple, and he's developed the Steelers defense around that premise.
Why change now?
Want another reason?
Casey Hampton just signed a three-year, $21 million deal. That all but guarantees at least three more years of the 3-4, and I still wouldn't look for it to change after that.
Changing defenses is difficult. Usually going to a 3-4 is considered the most improbable, but I would wager that, by the evidence above, moving to a 4-3 would be almost deadly for the Steelers. They don't have the financial resources or desire to gut an entire unit and start over.
So enough with the 4-3 talk. Enjoy the 3-4. It's much more exciting to watch.