Oh, I won't argue the economics one bit. Indeed, that's been at the heart of the AFCI debate here!
Originally, they were to allow AFCI devices, so you could handle things as you did with GFCI protection. When the requirement actually took efect (1999), though, the code was suddenly re-written to proscribe the devices. Later editions have made devices acceptable under certain restricted situations .... but the device makers have not made the devices available. Once bit, twice shy! For the guy in the field, it remains a puzzle how a 'protection' available in the device costs only $10 extra, while it costs at least $35 additional if the protection is placed in the breaker.
There are any number of reasons why MWBC's are used ... what is worth noting is that our most recent edition (2008) surprised us with numerous new restrictions upon their use. The next edition has several proposals to restrict them even further. Much to my surprise, one breaker manufacturer (Square D) is leading the charge, with a published opinion that MWBC's ought to be eliminated altogether. Personally, I cannot understand why Square D is concerned about this design issue.
