If there's no market for specialties where you're going to teach, it's hard to justify the expenditure. One alternative is to self-certify as MSDT once you've certified 25 students, assuming you have experience in the specialties you plan to certify for. In other words if you are a dry-suit diver and have 20 dry suit dives and have certified 25 students, you can apply to PADI as a Dry Suit Specialty Instructor. This holds true for most of the specialties. As Andy/DevonDiver says, you will most likely pursue Specialty Instructor Status in the five specialties you find most useful and interesting. If your shop doesn't market MSD training, maybe once you're an MSDT you can change that to bring more business to your shop and earn more yourself by teaching this courses.