Dear Rfrancis: Core biopsies are accurate provided they obtain the tissue in question. Since mammography is used, the doctor can tell if the needle is positioned correctly into a density. If calcifications are being examined the sample can be validated by the presence of calcium deposits within the sample. Since you had this already, it makes sense to see what comes back and take it from there.
It's partly a judgement thing: a core biopsy is very accurate in terms of the tissue removed -- for example, it's much less subject to error than a fine-needle sample (which is highly accurate in the right hands [or eyes!]). The only issue is the extent to which there could be doubt that it sampled the correct area. In a very large lesion, the question might remain: well, the area we sampled is fine, but what about the area a centimeter away? That's where the judgement comes in: if the area is pretty uniform, then a core sample ought to be highly representative.