Here are the 3 game statistics for passes intended for Santonio Holmes:
Here are the 3 games statistics for passes intended for all other receivers:
The stats largely speak for themselves, but since interceptions are such non-repeatable phenomena (i.e. the stats don't tell you about hail mary's, missed reads, etc), I'll look at each of them specifically. Roethlisberger has thrown 4 interceptions on the year. One was a hail mary at the end of the half in the Tennessee game ("intended" for Mike Wallace) that no one should take the blame for. One was on the deep ball to Wallace; we can argue what would have happened if Ben hadn't been hit or that Wallace didn't adjust to the ball very well, but it's hard to say that interception is primarily on anyone other than the blocking, specifically Max Starks, who got beat inside when he had outside help. The only interception that was clearly completely Ben's fault was the pass he overthrew to a covered Holmes in the Tennessee game. The fourth interception was in the Cinci game when Holmes missed a hot read and ran his normal route instead of a slant. So that's 1 interception on the QB, 1 on Holmes, 1 on the OL, and 1 dong interception.
I'm often hesitant to conclude very much from football statistics ("by employing linear extrapolation we can tell that Roethlisberger will put up a -634.7 passer rating while passing to Holmes in week 15"), but it's pretty obvious that Holmes was absolutely wretched in weeks 2 and 3 and a big part of why we lost both games. It should also be noted how positively Reggie-Wayne-like he was in week 1: if you take away the interception that was all on Ben, he gave his QB a rating of 146.6. He's one of the main reasons we won that game.
In my opinion, this offense can only be great if Santonio Holmes is great. The rest of our roster of pass catchers is filled with upside guys, complementary pieces, and veteran leadership. Ward, when he's not committing "pass interference" (emphasis on the quotes), can still get open, but he's no longer a big threat with the ball in his hands. Miller is a similar threat. Moore is a great outlet receiver, but he's not the centerpiece of an offense. Wallace is extraordinarily promising, but, like the rookie he is, he still misses downfield blocks and steps out of bounds for no reason. Sweed can apparently get separation anywhere on the field but even drops the balls he catches.
Holmes is the only player on our roster who is ready now to be both a go-to-guy and a scoring threat on any given play. Who else on the roster could catch 9 passes and yield a 11.9 YPA in a given game? Who else can attack all levels of the secondary like Holmes? When Ben has thrown to receivers other than Holmes, he has an amazing 80% completion percentage, but that has only given him a much less amazing 7.9 YPA and a downright boring 10.0 yards per completion (Holmes has a career 16.5 YPC). Wallace, or even Sweed, could turn into a threat like Holmes, but they're not there yet.
I have no idea if Holmes' problems over the past 2 games are simply a lack of concentration, the result of a vague wrist injury, or something completely different, but we need Holmes to be "great", like he was in the playoffs last year and like he was in week 1. The defense may not be quite as good as it was last year, but when Polamalu gets healthy, it will be fine. The offensive line has been better than expected (better than I expected, anyway), Roethlisberger has been outstanding, and even the offensive playcalling has improved some over last year. The missing piece (other than a miraculously resurrected running game) is a consistently dynamic playmaker on offense that can score from anywhere on the field. If Holmes can fill that role in the offense on a week to week basis, we can play with any team in the NFL. If not, however, no one else is ready to step into that role and we'll continue to struggle to put teams away.