I once asked for help with getting the vocabulary name to appear as the top level in the breadcrumbs, but of course it's not really itself a term, and the breadcrumbs only return terms (i.e. categories) in 4.6. So the offered solution was to put another category immediately below the root and put the whole taxonomy under this term. This then appears in the breadcrumbs as the top level next to home. Perfect. And with taxonomy context, you can even get all the subterms to appear as a nice category list.
However...I've now hit a serious problem. I have five vocabularies and each have large taxonomies. The largest has about 700 terms (jobs) arranged into numerous subcategories. There is significant overlap in the meanings of the terms and so many terms sit in one, two, three and even sometimes four categories. This is not a problem if one uses multiple parent. In fact, that is exactly the advantage of Drupal over other CMS in that it allows that typically semantic overlap in designing a taxonomy.
But, and this is a big problem, in order to get the top category to appear in the Breadcrumb, a top level term had to be used. With multiple parent enabled, every time it pulls a term from the vocabulary, it also pulls this top level term and puts it into the breadcrumb. If a term appears under three subterms, then it repeats the top level term three times in the breadcrumb. With multiple categories already appearing in the breadcrumb, it starts to look cluttered and when you realise there are repeated terms, it starts to look very unprofessional.