I think the only benefit to updating them regarding your whereabouts is you prevent them from obtaining a default against you.
After a collection agency files a lawsuit they are required to "serve" the initial lawsuit documents on the party they are suing. To "serve" documents on the party, they either have to hand deliver them to the party or leave them at the person's home address. Debt collectors can be pretty awful, and I've represented a couple of people that were not living in the country at the time a debt collector claimed to have "served" them with the documents. If you let them know you're not in the country then they cannot go to your old residence and leave the lawsuit documents and claim they "served" you. If they serve you with a lawsuit and you don't respond, then they can obtain something called a "default judgment" which basically means they prevail without having to prove their case. However, if they do obtain a default judgment and you were out of the country there are still mechanisms that you can utilize to challenge the validity of the judgment, but it may be best just to avoid the hassle by letting them know you are no longer in the country.
Messrs. Lyle and Hinz make some excellent points. My general rule of thumb is that, unless there is a distinct advantage to you, don't give a debt collector any information. The devil is in the details, however.
The legal analysis of any situation depends on a variety of factors which cannot be properly represented or accounted for in a response to an on-line question. Any answer, discussion or information is intended as general information only, is not intended to serve as legal advice or as a substitute for legal counsel, and should not be relied upon in making any decision. If you have a question about a specific factual situation, you should contact an attorney directly.
If you are out of country, the statute of limitations will be "tolled", but the creditor isn't likely to know that and likely will write off the debt after a few years. If your whereabouts are unknown, a creditor can obtain service by publication of the summons.
No one should rely upon generalized information for their specific matter unless they have consulted with a qualified professional specific to their matter. This is not intended to be used as legal advice to persons who have not retained my services in writing.