A lot of the discussion seems to assume that a given developer has a constant level of productivity and hence, can be salaried according to that level.

I'd say a developer's productivity can vary according to motivation; who they're working with; how interesting the work is; whether they're well managed or not...

So, while salary is a linear measure, it makes no sense to me to assume that productivity can be measured the same way.

I'm fairly sure my own productivity can vary by as much as an order of magnitude according to circumstances. Wonder what I should be getting paid? :)