You shouldn't lead with your team's main sweeper, or it runs the risk of dying early. Sweepers switch in on resisted or immune attacks, such as Heatran to a Fire attack. Thee are some sweepers that are so fragile they can't switch into anything, such as Deoxys-A. These require a sacrifice to get on the field.
For more, go through the RMT archives. Hope this helps and good luck!