Why buy protocol analyzer modules for an oscilloscope?
I think the main benefit is that you can measure analog signals directly correlated in time to decoded serial protocol traffic in an integrated display. 

These types of modules will also often come with an indexed search kind of capability to let you jump to user-defined events through the common interface of the scope. Sometimes they will also identify errors or warnings in the serial traffic as well. Lots of bells and whistles beyond just decoding the serial traffic for you.

For a very large company that bills engineering hours at say $100/hour they can justify such a capital expense if the features of the module will save 7 hours of labor (debugging time) over the useful life of the scope/module, as compared with the doing that same labor without it. The calculus works out, since these companies are obviously able to sell them.

Whether that kind of thing is worth $700 to you is another story though.