The Federal Aviation Regulations say you must accept a clearance given by air traffic control to another pilot. Here is the link to FAR 91.123 para. (e) that talks about the restriction. There is nothing in the regs that says you cannot listen to a clearance given to another pilot, but why should you? You have enough going on in your own cockpit without having to worry about what the next pilot is doing.
By all means, if your cockpit workload is high, concentrate only on what you are doing. If you have a second, it can pay big dividends to listen to what is happening to other pilots sharing the same radio frequency. Why? In many cases, whatever that other guy got from ATC is going to happen to you as well. Let me explain.
When a piece of airspace fills up, and a controller is very busy, it is likely that all flights on a particular airway, a published departure or arrival procedure, or in an airport traffic pattern are going to be cleared to do the same thing. If Pilot A was directed to do X, then Pilots B, C, D, and E are likely to be cleared to do X as well.
That makes sense, doesn’t it? If you are busier than a one-armed paper hanger, would you have time to invent new and creative ways to hang paper? No, you would use the tried-and-true, standardized procedure for getting all that paper to stay on the wall.
What does all this mean to you as you are flying along in flight? If Pilot A is told to slow to final approach speed 12 miles from the runway, and you are following Pilot A, guess what you are going to be told to do 12 miles from the runway?
Listen to the radio. You can gather all kinds of information that will help you fly a more efficient trip, possibly saving time, money, aggravation. Best of all, that information comes to you free of charge.