I think I come down on the side of those who create new species until sufficient evidence can be found to merge them. It strikes me as better science — a connection is, to my mind, more significant than a lack of a connection.
That is an extant fight in taxonomy, and one which has been going on for more than a century and a half: http://en.wikipedia.org/wiki/Lumpers_and_splitters
> splitters make very small units – their critics say that if they can tell two animals apart, they place them in different genera … and if they cannot tell them apart, they place them in different species. … Lumpers make large units – their critics say that if a carnivore is neither a dog nor a bear, they call it a cat.
Fundamentally, there is no obvious and unambiguous definition of "species". It is a functional definition that is highly useful, but it is not exactly predictive is every case under the sun. There will always be confusion on the margins, because the natural world is sometimes messy that way.