Good point.

More generally as per the wikipedia article http://en.wikipedia.org/wiki/Hyperbolic_discounting#Criticism exponential discounting is only correct if you are equally certain of the payoffs at all the different times.

More broadly it assumes no model error. Whatever decision model you are using you need to be 100% certain of it to justify exponential discounting.

Nassim Taleb points out that quite a few alleged biases are actually quite rational when taking into account model error and he includes a derivation of why the hyperbolic discounting formula is actually valid in many situations.

Silent Risk Section 4.6 Psychological pseudo-biases under second layer of uncertainty. Draft at http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2392310