I'm no repellent or bug expert, but a quick Google search turned up an interesting DEET hypothesis or two.
Research says that higher concentrations principally provide protection for longer. Found one claim that the duration is proportional to the logarithm of the concentration applied. So you would need to apply less often.
Also, one could theorize that the "other ingredients" in sprays are something that evaporates, like alcohols or similar. If this were the case, and you apply to skin, then all of the DEET and only the DEET in a given concentration would be on your skin, right? Meaning the only thing more dangerous about 100 percent DEET versus lower concentrations would be the ability to put on more per each application.
Anybody know if this is how it works?