he says that things matter because people care about more than themselves. so if you don't care that people find porn on your PC, then it doesn't matter - if you do care, then it does.
to answer your original point, the answer to whether it's irrational to care about the the world after your death, comes down to whether there is a rational basis for the world being in some state. so if it's rationally verified that people not dieing from cancer is a good thing, then working on a cancer cure that won't be completed until long after your death is rationally time well spent. if you don't care about other people, then there isn't much reason for you to be convinced that it matters. whether or not you would be wrong, well, i wouldn't know. do you want to be convinced one way or the other?