Adding Users To A Role In Bulk

I have a need to add all the users listed in the sysxlogins table to the db_datawriter role. I wrote a proc that does this. It indicates that each user is successfully added to the role, but they aren't. If I look at the user in EM, they don't have that role checked. I've tried all the obvious stuff like close EM and re-open, etc...it doesn't help.

Here is the proc
CREATE proc sp_MyProc
@DBName varchar(256)
As

Declare UID_Cursor Cursor
For
Select Name from master..sysxlogins Where Len(Name) = 7 Order By Name

Open UID_Cursor
Declare @Name as varchar(256)
Declare @TempString as varchar(8000)

Fetch Next from UID_Cursor into @Name


While (@@Fetch_Status <> -1)
Begin

Set @TempString = 'sp_addrolemember ''db_datareader'', ''' + @Name + ''''
Exec(@TempString)

Fetch Next from UID_Cursor Into @Name
End

Close UID_Cursor
Deallocate UID_Cursor