> Are depositions ever really useful?

Absolutely. Many depositions (and in significant cases, nearly all of them) are videotaped; then for trial, each side creates a "documentary" consisting of selected excerpts — think of it as being like a segment of a news show such as 60 Minutes. A string of "I don't know" or "I don't recall" answers, or even unduly-long pauses before answering, can seriously hurt a witness's credibility.

And if a witness testifies differently at trial than s/he did at the deposition, on cross-examination the opposing counsel will pounce and play back the relevant portion(s) of the video recording to impeach the witness's credibility that way as well.

Finally, depositions are very often used to "prove up" basic facts, as 'rayiner said in the GP.