There are no drawbacks of Names Server.Oracle has introduced Names Server because of its advantages.The main advantage of Names Server is to eliminate local naming and failover.
U may be needing Names Server when u have many clients and making changes to tnsnames.ora file when adding new database is cumbersome.In Names Server there is a server in which names server is created and the clients gets connected to Names Server for name resolution.There is a elimination of tnsnames.ora file on each client when names Server is used.Another advantage of Names Server is failover.In ur network there can be many names server so in case one names server is down for any reason ,clients can still easily connect to the database.
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The only thing to think about is that Oracle Names is going to be replaced by Oracle Internet Directory which is free for use as a Names Server. So if you are researching, do it with Internet Directory.