This study seems to be the one in question. It's only a Phase 2 trial and I think it's perfectly reasonable to ask the doctors "what's the benefit for me?". I'd worry that delaying a stem cell transplant to take SGI-110 in this dosage-testing trial might not be in your best interests because the longer you wait, the higher the chance that you'll evolve to leukemia.
A second bone marrow biopsy could give you the assurance you want that the previous lab results were valid. It could also reveal spot other chromosome problems that happen to show up in the cell sample. The question to ask yourself is what would change if the results differ in some way from the first bone marrow biopsy? If the results won't change your plans then there's no reason for the test. If they WILL help you make a decision, it makes sense.
Labs won't all produce the same analysis of a given sample, but suppose they don't spot the chromosome 7 problems. I don't think the doctors would call it spontaneous remission. Instead I think they'd conclude that the second sample just didn't happen to include those bad cells. Labs might not always spot a problem that's present, especially due to sample variations, but I doubt they could have reported bad chromosome 7s if there weren't bad chromosome 7s.
The disadvantages of another bone marrow biopsy would include the cost (maybe not covered by insurance this time) and the discomfort, and a very small risk from the procedure itself. Those don't seem like big disadvantages when you're trying to save your own life, but in my opinion the main reason for another biopsy would be to see if a treatment or transplant has had the desired effect, or if your bone marrow has changed over time.