I agree that it is possible that there is life somewhere else in the universe. But I don't believe that they have ever had any contact with planet earth, simply because I don't think that interstellar travel is feasible. Secondly, although people talk about unexplainable phenomena like abductions,
cow mutilation and
crop circles there are usually easy explanations that no-one talks about because they're not as entertaining.
In any case, I seriously doubt anything interesting is still going on at Area 51. With all the scrutiny they get there, don't you think they would have moved anything suspicious to another base? The air force probably encourages the rumours as a decoy so that people flock to 51 rather than where the action is really happening.