njhornung, I completely agree that the goal of treatment is not to reach values in blood test, but to prevent fractures.
The sad truth is that for all of these medications there are patients for whom the drug is not effective. So, the purpose of the serum bone markers is tell the patient the the drug is or is not working. A test before and a test one or two months after beginning a drug treatment can prevent fractures for these patients. To be on the verge of fracture and to remain so for 18 month or 2 years on a medication that is ineffective puts a patient for 18 months to two years of additional risk, not to speak of the often endured side effects is not advantageous in any way, even financially.