The issue here is not the use of BMB but of the need to do two at a time (one in each hip). Many malignancies are "focal" in the sense that they are not evenly distributed throughout the marrow space so that it is possible - even likely - that a single biopsy might miss a diagnostic area. Probably the best example of this is multiple myeloma in which it is possible to miss a place totally involved by the myeloma and pick up cells that appear to be perfectly normal and free of disease.
I believe that what is being said here is that multiple same day biopsies do not provide any additional information and that saving pain/aggravation/time/money/etc. is justified in CLL
When you were first diagnosed, a bone marrow wasn't really necessary since the presence of the abnormal clone of cells could be identified from the peripheral blood. Now that you can apparently cleared your blood and perhaps lymph nodes of these cells, the question is where to check to make sure that this remission is a good one.
Leukemias arise in the marrow so it makes sense to see what is going on there. As a test, PCR can be done on any bodily specimen except red cells (they have no nucleus). One of the other advantages of the marrow specimen is that you get a LOT of cells to test so the chances of missing one or two "baddies" is much lower than if you used a smaller number of cells.