On the Random Topics blog, I said:
On the TalkLeft discussion of this issue, cboldt somewhat disputes my opinion that Kasper will determine the issue.
The point I think he's making is interesting. If Crump were on the defense's side, then on direct appeal the defense could proffer his testimony, allowing the appellate court to determine if the error was harmless. Since Crump is an adverse witness, there's no way to really know how helpful his testimony would have been. So if there was an error, the harmfulness would be difficult to determine.
The problem could be solved by granting a new trial if denying the Crump deposition was an error, but that seems like a roundabout way of handling an error which might or might not have been completely harmless.