The May, 2007, issue of the Illinois Bar Journal includes an article titled "Oppositional depositions - telling your client not to answer," by Helen Gunnarsson (for-pay link, I think).
In the article, Gunnarsson explores the recent federal case of Redwood v Dobson, 476 F3d 462, which I wrote about here, and analyzes similar issues under the Illinois rules.
She kindly talked to me for the article, asking how I would approach questions in a discovery deposition that I didn't think were designed to lead to discoverable information. Would I instruct the witness not to answer, or would I just keep my mouth shut, waiting for the 3-hour time limit to expire? My answer--
Schaeffer says that these issues seldom come up in his practice because "the lawyers keep on track." But if he were presented with a question from the other side that he felt did not meet the standard for discovery or evidence, he opines that in a state court case he would instruct a witness not to answer.
"I'd always do this before deciding just to let the clock run out - lots of mischief can be accomplished in three hours." Schaeffer comments that "even in a close case, the other lawyer is unlikely to file a motion about the question you've just instructed a witness not to answer. It's a combination of knowing they were close to the line of a permissible question, they don't really need the information, and they don't want to bother with a motion."
There's more detail in the article. Anyone interested in the application of the Illinois rules to the problem presented in Redwood v. Dobson probably has access to the article as an ISBA member.