Framing a situation as an economic transaction has been shown to interfere with other motivations. The classic research paper would be http://papers.ssrn.com/sol3/papers.cfm?abstract_id=180117 where introducing a fine to make parents pick up their children at daycare considerably worsened the situation. The interpretation is that behaviour that was evaluated in a context of prosocial motivations such as being decent towards the staff, keeping a good relation with them, etc, were supplanted by a view that 'it's worth the cost of the fine'. It is worthy of note that going to an economic reference frame is often easy whereas going from an economic reference frame to one based on social values is frequently difficult - it certainly can't be expected to recover spontaneously just because the money is taken out of the picture again.
Now, TOR is very much about prosocial values - liberty, free speech, the right to communicate without being unjustly surveilled, and so on, and because of that, I think it is potentially quite vulnerable to these kinds of effects.
I think there is a substantial chance that running a TOR node is not going to be worth the economic profit in the long run, but that shouldn't matter, because running a TOR node is about something other and something more important than making an economic profit. If we start framing it as a business transaction there is a risk we hurt the (in this context) much more important motivations to operate a node. I think the TOR initiative should be developed on motivation based in the human rights values it naturally supports. And in this case I believe the option of monetary compensation is genuinely at odds with that.
I'm somewhat sad to be saying this, because as I mentioned, it seems to me that the project is based in the best of intentions. Some components of the TorCoin system may be valuable for other service functions in the network.
I summary, I think the psychological effects of introducing the compensation scheme really should be carefully evaluated before any attempts to introduce it in practice. My spontaneous impression is that it is the wrong method of motivation for growing and maintaining the TOR network and should not be introduced at such. If it is after all introduced, that should be done in a manner that is informed of the psychological consequences.
As a preferred alternative we should emphasize the social and ideological benefits of TOR for core motivation and lower the threshold for contributing by making it technically trivial for the contributor and by making the situation as clear as possible about risks and benefits of operating a node.