Consider the following ordinary form of argument. Risk R1 is less than risk R0. Risk R0 is a reasonable risk to take. Therefore, risk R1 is as well. Typically, R0 is a risk that reasonable people routinely take, without worrying about it. Thus, we might be told that flying is safer than driving, and since (this premise is usually implicit) it is reasonable to risk driving, it is reasonable to risk flying.
This is a bad form of argument as it stands, for two reasons. The first is that it needs to be established that the benefits associated with taking risk R1 are no less than those of risk R0. The second is that even if the benefit claim is true, this only shows that it would be rational to take R1 instead of of R0. It does not show that it would be rational to take R1 in addition to R0. (Thus, it would be reasonable to fly regularly instead of driving regularly. But given that one is driving regularly, it does not follow from the risk-comparison alone that it is reasonable also to fly.)
Here's an example to show the second failure. The fatality rate for Mount Everest climbs is apparently about 9%. Let us suppose that Kenya is a single woman, with no family, and who does not do any job for which she is essential (e.g., she is not an irreplaceable top cancer researcher). It might (I actually doubt it) be reasonable for Kenya to climb Mount Everest, for the sake of the various goods instantiated by the climb, despite the 9% risk of death. But if the above argument-form were sound, it would be likewise reasonable for her to additionally engage in another activity that carries an 8.9% risk of death and has similar benefits. But the argument could then be iterated. If there was some third activity that carried an 8.8% risk of death, it would be reasonable to additionally do that. Therefore, by repeated application of the argument form, we would conclude that if it is reasonable for Kenya to climb Mount Everest, it would be reasonable for her to do climb Mount Everest and do A, B, C, D, E, F, G, H and I, each of which has a slightly lower risk of death than the previous. Let's say the risks are 9.0, 8.9, 8.8 and so on. Assuming independence (not quite right), her chance of surving all ten activities is less than 47%. But it seems that it is only reasonable to engage in a series of actions that one is less likely to survive than not for the gravest of reasons (such as saving someone's life). The sorts of reasons involved in the climb of Mount Everest are not like that, and even having the benefits ten times over is not worth it.
A related, but I think not identical, issue is that benefits need not be additive. The benefit of both A and B need not be the sum of the benefits of A and of B in isolation. It might be more, or it might be less.