SQL Users are still available primarily to give access from systems that don't have easy AD access. For example in my company our UNIX systems do not use AD so applications written on those systems need to use a SQL Login in order to connect to a SQL Server instance. I'm told it is possible to set a UNIX system up so that it can use AD but ours just aren't set up that way.
As far as copying the SQL Server logins/users from SQL Server auth to AD auth, I'm afraid it just isn't possible to do directly. In fact it isn't even possible to transfer a User from a SQL Server login to an AD login. You get the following error:
Msg 33017, Level 16, State 1, Line 1
Cannot remap a user of one type to a login of a different type. For example, a SQL user must be mapped to a SQL login; it cannot be remapped to a Windows login.
There are however a number of tools out there that will script out permissions and you can then adjust the script to use AD authentication.
For example at a database level you can use the built in tasks/generate scripts.
From there you can specify that you want to script user only permissions. You will end up with a script that looks like this.
/****** Object: User [UserName] Script Date: 10/19/2014 8:42:08 AM ******/
CREATE USER [UserName] FOR LOGIN [LoginName] WITH DEFAULT_SCHEMA=[dbo]
ALTER ROLE [db_datareader] ADD MEMBER [UserName]
From there just do a replace of [UserName] WITH [Domain\ADLogin]. You will have to create all of the logins manually but that's pretty easy too.
CREATE LOGIN [Domain\ADLogin] FROM WINDOWS
I do realize that could be a bit of a pain if you have hundreds of databases but if that's the case then you can use various on-line tools\scripts to find & generate your scripts for you.