I am not sure how there could even be a situation where the only thing going on was currency exchange. Even in the earliest days of Bitcoin when it wasn't being used for much of anything at all, people were still speculating on its future potential value. That is legitimate speculation and that is more than just currency exchange. I myself realized the potential value of Bitcoin early on and threw some money in at $1.44, saw it rocket it up to $30 and crash back down. I still have my original coins because I think Bitcoin has even more potential in the future.

But to go even further, even just straight up gambling is not a zero sum game. People gamble for a variety of reasons, but the bottom line is, they feel that on any given 'roll of the dice', risking their money at that point in time is worth it. Hence, even if Bitcoinica was straight up gambling (but I doubt that it was), it still wasn't a 'zero sum game' because the people involved thought that it was providing value to them at the particular time that they decided to use it.

As for a 'healthy' economy, I would personally like to see a lot of those things, but maybe the economy won't include much of those things. Perhaps Bitcoin will turn into some digital form of gold hoarding where people just buy them up to stash away some hidden wealth for a period of time. Who knows? In any event, no aspect of the Bitcoin economy, or really any economy is a 'zero sum game' as far as I can tell. People derive value from all kinds of crazy places, and if they are happy, that has all kinds of positive secondary effects.