> After all we have so many laws that offer similar type of protections already, such as a wife not having to testify against her husband. At some point the society decided that it's the "right thing" to do.

To put it in context: spousal privilege is rooted deeply in religious tradition going back hundreds if not thousands of years. It fell naturally out of a belief that people already had (the indivisible marital unit). That's what it takes to overcome the default presumption that "the public has the right to every man's evidence." Now, the fact that something has been the case for probably 800 years doesn't mean it can't be changed, but does suggest that if people really had a problem with it, they would have taken issue with it by now.