Why Visual Studio detects unused using directives?

First, sorry for my English if I do any mistake.

The point is, I've just installed Visual Studio 2015 RC and UnityVS Tools to change from MonoDevelop because my experience with it was awful. So now that I have my project correctly configured in VS 2015, I have noticed that Visual Studio detects some of my using directives as errors but these are marked as correct in MonoDevelop so I'm asking if this is a mistake in Visual Studio or it's just that with Visual Studio I have to remove the unused using directives to run my project correctly.

The lines that are marked as errors are these:

 using UnityEngine;
 using System;
 using System.Data;
 //---Errors---
 using System.Data.Odbc;
 using System.Data.SqlClient;
 using System.Data.OleDb; 
 //------------
 using System.Data.OracleClient;

I need them to be able to connect my application with different databases. Thank you in advance.

Edit: The link that @maccabbe has provided has helped me finding the solution. Seems like, in order to use the System namespace in Visual Studio, you need to add the System.Data.dll file from the MonoDevelop directory.

But even if you have included it in your project, Visual Studio can fail sometimes finding it, so be sure you have the correct version of the dll (I think it's 2.0).

Apart from that, in Visual Studio it's unnecesary to write subnamespaces if you include the namespace parent unlike MonoDevelop which needs to include all the subnamespaces used in the script.