Hello ! I think time issues are far from being the only advantage at being pessimistic when developing software... It also allows to have a quite accurate (but nonetheless still optimistic) preview of what could go wrong when the software runs (and then helps preventing it), how the users could use the software the wrong way, how hackers could hack,...
Murphy's law states that everything that can go wrong, eventually will. Well, for software development, it is an understatement : it will be even worse...