First, let's discuss why sacks are good. One reason, they end a passing play. Well, what about run plays? You need to end them too don't you? Passing plays only account for about half of the snaps in a game, so are we just going to forget about the other half of the game waiting for a sack? If you got one sack per game you'd finish the season with 16 and probably go to the pro bowl. That's 16 plays out of about 400. Pretty small impact overall.
So, back to Daniels. He had 9 passes defensed last season, 2nd best among all DL's in the NFL. You know what that does? It ends a passing play. That's right, just like a sack. Daniels had more PD's than all of the other Redskin DL's combined. Hell, he had MORE PD'S THAN LARON LANDRY!

Now let's not forget his versatility. he routinely moved inside on passing downs so Wilson or Washington could pass rush from the end spot. Think that might have something to do with his sack numbers? He spent many 3rd downs, where you knew it was going to be a pass, rushing from the inside DT spot!
Now effectiveness against the run is harder to judge because sometimes you make the play by plugging a gap or backing the OL into the backfield and stats don't reflect those plays, only tackles are recorded. But watch games and listen to what other players and coaches say and you'll see Daniels is stout against the run.
Lastly, I'll point out his leadership. He is a bona fide locker room leader. he's also known as a leader in the weight room where he is still the strongest player on the team. in fact, this off-season he took two weeks off and then immediately started competing on the power lifting circuit. He recently squatted an amazing 722 lbs! and says he is stronger now than he has ever been and feels quicker off the line as a result. A video of him power lifting is linked below. So for those you doubting Daniels contribution the the Redskins franchise, please consider more than his sack stats before making up your mind.
http://www.powerliftingwatch.com/node/8535