Regarding multiple citations for a fact -- here are my thoughts -- and I am not an expert.
The main disadvantage, as already noted, is when printing trees with sources. You can get many sources for "sex". I think one source is good enough for "sex". You can get many sources for Name. If the name is always the same, why repeat them? Of course, names are often not the same -- so in those cases you will want multiple facts, with a source for each fact (or use the AKA feature). Also as noted previously, data closest to the fact may be the most accurate. So if I delete census sources, I delete the ones most remote from the fact (in terms of time). If you end up with multiple sources when merging information from Ancestry.com you may have to manually delete the unwanted links (like for "sex").