This is the thing - you say yourself that the Greeks are in "dire straits financially", and you are completely correct.
Given that fact, how on earth would they expect to care for, conserve and properly display the marbles? Granted there is the remodelled Acropolis Museum which is fairly up-to-date, but you need to staff it and maintain it. Our major museums are struggling enough already, and the UK is in a considerably better state than Greece is.
The argument over the ownership and location of the marbles is of course much, much more convoluted and complex than that, but if I was their curator at the BM I would want considerable proof that Greece can care for them properly before any negotiations can begin.
I also agree with Tobyjuggs argument. I love the fact that I live in the UK, yet I can study collections (ceramics and stone, in my case, although nothing quite so fine as the Elgin Marbles!) from all over the world. I love the fact that I don't have to go to France to see French art, and that I don't have to go to the US to look at Native American work. I love the fact that I can take my little cousin to learn about ancient Egypt and Rome without having to book a plane ticket. I love the fact that I can AFFORD to see them. If we only have our own country's artefacts in our museums, we lose so much (this goes both ways, by the way, we need to loan objects abroad too!).
Personally, if it was up to me, I would have the Elgin Marbles on a worldwide tour, ending at Greece, where they would stay for a long-term loan (5 years?) with a very, very tight contract. Sadly I suspect however that if they were loaned to Greece we would never get them back, despite hefty legal agreements.
I do think the decision to loan some of the marbles to the Hermitage, fantastic facility though it is, at this point in time was a bit insensitive.