I too have a problem if they are not informed within some period of time (days or weeks, not months). But I can see some situations -- admittedly not many -- in which it is reasonable in light of all of the facts and circumstances of a particular case to delay a few days before informing the target of the search.
Consider the following example. Suppose we believe that person X is a member of a terrorist cell. If we search his laptop for evidence under a traditional search warrant, he will immediately alert the other cell members and they will disappear before we can discover who they are and arrest them. Under these facts, I think it would be reasonable to get a warrant to search his laptop without letting him know for some short period of time -- which we would use to arrest the other cell members using the information we retrieved from the laptop. The short delay hasn't prejudiced him in any way, but it enabled us to make the arrest that we otherwise would not have made.