I would imagine that a fair number of the snaps are "empty" in that the play goes downfield or to the sideline where the secondary makes the play or it ends in the backfield with a sack or incompletion. But I don't think the snaps are insignificant -- there are draw plays, screen plays, short slants, underneath routes, etc -- that the linebackers clean up on.
The number of snaps is meaningful. I estimated teams spent 40% of the time in one subpackage or another passed on run-pass data from Doug Drinen's data dominator down and distance logs three years ago. Multiple coaches have said that their base defenses are only on the field about half the time in recent seasons. The data we have on Curtis Lofton from the 2008 season suggest he played approximately 60% of his team's snaps last season.
Some additional food for thought from recent seasons:
Andra Davis (2005-2006 as every-down ILB in Cleveland) -- averaged 89 solos per 16 games
Andra Davis (2007-2008 as base defensive backer only) -- averaged 57 solos per 16 games
Leon Williams had 30 solos last season in a platoon role (nickel only) with Davis
Bradie James second half split from 2008 was significantly more productive. I think there's something to the more aggressive philosophy that occurred when Wade Phillips started calling the shots, but I think that production was primarily driven by James' increased snaps on passing downs that occurred at about the same time.
Jeremiah Trotter v2004 was on pace for a 73 solo tackle season in a base defensive role with some nickel duty. Trotter v2005-06 in an every down role had 102 and 88 solos.
I've seen arguments elsewhere that players don't get enough third down snaps for the nickel packages to be worth considering. That clearly severely underestimates that number of first and second down snaps defenses spend in the nickel (and dime). And the instances above support my argument that there are plenty of plays in the middle of the field to provide tackle opportunity for linebackers.
I agree that it's too superficial to simply project out the percentage of a player's base defensive solos to a full time role. Projecting Lofton to 112 solo tackles is certainly pushing it. In Lofton's case, I think he's likely to be productive enough in coverage to leverage those snaps into what guys like Leon Williams and Bradie James and Lawrence Timmons have been able to do in those packages. By comparison, I was never as excited about the prospects of a guy like Daryl Smith in an increased role.
Timmons himself is an interesting case.
I really like Timmons. I was happy to leave him ranked highly as a rookie in anticipation of him earning an every-down role. He's clearly looked good in limited time and looks to have some big play value, too. But it's harder for me to project Timmons to 92+ solos than it is Lofton to a big number. His surrounding cast is more likely to limit opposing offenses and less base defensive snaps may be generated. The Steeler defenders have had some of the worst tackle opportunity in the league for a number of seasons now. And Farrior doesn't get much credit as a tackle sponge because of the relatively poor opportunity.
It goes against my "go for upside" nature, but I think the chances that Timmons finishes with 75-80 solos is much higher than the chance he finishes with 90 or more. Still, for consistency, my rankings should reflect Timmons' upside given my current rankings of guys like Lofton, Justin Durant, DJ Williams, and others. I'll be moving him up in the next update.