Fans of demurrage allege that it is unfair that you have a future time preference while someone else has a now time preference. Because you are capable of planning for the future, you must be punished with a currency that loses value over time, so that you can be encouraged to be as wasteful as everyone else.
While this does seem to be what they have in mind, I really don't see how demurrage changes anything with regards to interest. As I understand the concept, demurrage means that you lose actual currency over time, as opposed to inflation, where the amount of currency remains the same but the value decreases. In my example, however, no one was actually holding currency for any significant length of time. Either I spent it immediately on capital, or I lent it to someone else who would in all likelihood spend it immediately on a different sort of capital. Either way, neither of us is affected by the demurrage.
Demurrage and inflation both have the effect of driving people to get rid of the affected currency as quickly as possible in favor of more stable assets. The advantage of demurrage, to the extent there is one, is that it is neutral with respect to things like interest rates--it doesn't create the same calculation problems as forced inflation. However, Impaler seems to be claiming exactly the opposite. If you want to make people overvalue future goods relative to present goods (i.e. eliminate interest), inflation is a much more effective tool.
As for time preference being "irrational", I would like to see Impaler put off all consumption for a year (or even a week!) and report back on whether future goods are just as valuable as present goods.