Some of the nastier scripts out there nowadays aren't really about gaining elevated privileges. Some of them, like the encrypting ransomware requires no special privileges at all, but simply access to user files, and to network files that the user has read/write access to. So while the critical aspects of a computer or a network are protected by execution and system resource access limitations, you need to prevent execution of unauthorized scripts completely.
I have to admit I've found signing Powershell scripts to be a mighty pain in the arse, but it does provide some protection against external scripts running when you maintain the blocking of execution of unsigned scripts. It isn't a complete protection, unfortunately, and Powershell is only one route by which this kind of ransomware could end up on a system. Vulnerabilities in Java, MS-Office files, and even the execution of Windows Scripting Host files (vbscript and jscript) seem more common from my experience.
The one bit of ransomware I saw got loose through a vbscript file attached to an email. For whatever reason, Outlook allowed it to be executed, and the user clicked the dialog that might have prevented it, and then the script went to town encrypting files on the user's own folders and the share. Fortunately there's a good backup regime in place, so there was very little actual loss, but it demonstrated that along with some vulnerabilities in Windows' execution protection schemes, the real weak link as always is users themselves.