Because otherwise you're risking a char you care about. Only play with a char who can do concentrated melee damage, avoid a 4 player table if you can, and if you do decide to use your own char try a 4th level since it really seems written for 4-5, not 2-4. If you are in a group of six, try five war priests and then it won't matter that the swashbuckler or investigator aren't especially functional, so the 6th person could try out one of them.
Brought a friend who is new to PFS but not at all new to RPGs ( seemingly the target audience for free RPG day) - didn't go well but at least he just lost a pregen. Yay, pregens.