It really depends on who your intended user is. If the slides are intended to form a presentation (quite likely), then the user is often the person making the slides in the first place. In such cases they're not a usability disaster at all. Rather, they're really helpful (being able to switch between different sections, or miss sections that your audience already knows about easily is really useful).

On the other hand, if the slides are intended for reading, then yes, they may be a usability disaster. But then, slides aren't going to be the best format for reading anyway.