Sign in as a Different User is removed in SP2013 to add it to the menu again a change have to be done to the Welcome.ascx page.
If you are change the SharePoint site as administrator there are things that can not be testet and don't look as the normal user will see the page. On this condition you will like to change user many time on you work.
In SP 2013 you can't signout without makeing som changes to the standard welcome.ascx page on SharePoint.
To put back the Sign in as a Different User do the following:
1.) Locate the file Welcome.ascx in the folder \15\Template\CONTROLTEMPLATES
2.) Open the file in a text editor and add the following code between the ID_PersonalInformation and ID_RequestAccess tag.
<SharePoint:MenuItemTemplate runat="server" ID="ID_LoginAsDifferentUser"
3.) Save the file. This should be done on all servers on the farm.
Now it should look like this: