I understand that it's just something you thought of off the top of your head and I agree that eventually we will get there. However, if you go a little deeper than off the top of your head you begin to see social, fiscal and logistical problems with it, and on that basis I don't think we're quite ready for it yet.
No system is entirely perfect, there's no point disregarding something cause you can see a problem. There are huge problems with cash payments and we don't just abandon that. This is still at a concept stage, not full specification. That said it's still interesting to have these problems pointed out so we can get ideas and solutions out there!
On the points on homeless people and tourists, there are a few things to consider. Today smartcards can be made for pennies, it's not unreasonable to think that homeless charities or welfare offices could supply the empty 'digital wallet' to homeless people at a low cost. As a poster above said you can easily hang on to them for many years, so it's likely a one-off cost. On the point of tourists, as cards could be so cheap it would be possible to provide them at foreign exchanges or shops, like the current 3V card. If following the wallet concept there would be no need to have a bank involved. Many phones in the next few years will have NFC as standard. It's conceivable that the tourists would already have this technology on their phone, so that could be used to access the manage the card/transfer money from person to person. If not, it's possible that a device could be rented from a foreign exchange or something that could provide this function.
Still I think as long as the rest of the world deals in cash there will always be a need for small cash payments, but it's very possible that this could be in very rare cases.