I am sure I am not able to answer this question, but It is a valid question to ask and for medical experts to debate. Surely the decision should be made based on risk vs benefits. Taking medicines long term for preventive reasons is not without risk. Taking some medicines can place patients at higher risk for health problems than the problem they are attempting to address or prevent.
A new study published in the New England Journal of Medicineprovides useful information for both patients and their physicians. The study followed women after an initial normal bone density test to determine the likelihood of them developing osteoporosis. It found that that "osteoporosis would develop in less than 10% of older, postmenopausal women during re-screening intervals of approximately 15 years for women with normal bone density or mild osteopenia, 5 years for women with moderate osteopenia, and 1 year for women with advanced osteopenia"
I believe this finding is important since it provides some real information about a persons risk of developing Osteoporosis later in life and will help physicians decide whether they should be prescribing bisphonphonates for particular patients.
An article in the New York Times, published on January 18th, discusses the significance of the above mentioned study and provides further information and is probably worth reading. It reports that "experts also generally recommend that most people on bisphosphonates take them for just five years at a time, followed by a drug holiday of undetermined length. The idea is to reduce the risk of rare but serious side effects, including unusual thighbone fractures and loss of bone in the jaw."