Is bookmarking a compulsory feature of SCORM 1.2? Is it a must or is it a 'nice to have'?
This depends entirely on who you are... If you are the LMS provider, then, yes, you must support the use of the "cmi.core.location" data model element. (This is generally what people use for bookmarking.)
If you are the content provider, however, then you are under no obligation whatsoever to use the data model element. Content providers are offered many tools via the SCORM data model, and they are obligated to use very few of them.
This, of course, speaks only to the technical conformance of a piece of content (or an LMS). Common practice indicates that most pieces of content should make use of bookmarking, as it is a straightforward addition and a useful one.