In OD&D, it was not unusual for one player to run multiple pc's (as many as 6 pc's, if there was just one player).
In AD&D, it became a bit much to run more than 2 pc's, unless they were all fighting types.
In 3E, we've tried running multiple pc's, or having a player run his/her character and one npc, and it's been difficult. In addition to the roleplaying problems inherent in running two characters, 3E is much more complex than the previous systems, with many more options. Only the most experienced players can do this well, and it should always be the player's choice, i.e., never force a player to play extra characters.
If your players can handle multiple characters, I would agree with those who've recommended giving them the Leadership feat for free and allowing each a henchman. If the primary character dies, and is not raised, give the player the option of creating a new character, or changing the henchman to a player-character - which could have interesting roleplaying possibilities.