How would you measure a favour as a unit of currency? Time would have to come into it, but then there's also skill and difficulty. It would be different getting a master tradesperson to tile your bathroom than getting your two mates to do it for their first ever attempt. And one hour of a really tricky problem is far different to one hour of mindless repetitive labour. You probably can't quantify it adequately in the end. It has to be negotiated each time between the parties involved in a trade.
Mokalus of Borg
PS - It might be one way to try and build a cashless economy, though.
PPS - Well, until we start printing generic favour coupons to pass around and keep track.