You still have to implement queries, sorting, joining, etc. If all your queries are simple key queries then you're basically done. You can also hash at the network level by assigning group hash identies to specific machines in your topology.
Where you will find a benefit to using a RDBMS is in heavy correlation, multilevel sorting especially when you're talking about very large data sets. This is one of the key problem that RDBMS address. If your requirements also say anything about atomicity and integrity of the data it may be a better investment in the long run to go with an existing database solution that had a couple decades to work out those details for you.
Where you will find a benefit to using a RDBMS is in heavy correlation, multilevel sorting especially when you're talking about very large data sets. This is one of the key problem that RDBMS address. If your requirements also say anything about atomicity and integrity of the data it may be a better investment in the long run to go with an existing database solution that had a couple decades to work out those details for you.