Okay, lets use Soarin as an example. There are what, 108 seats for each show (three rows of twelve, times three sections)? So, youre number 100 in line. That means youre included in the next pre-show group. But wait! Ten of the parties ahead of you each happen to have sent one person to Mission: Space to get FastPasses and they each rejoin their respective parties while youre waiting. Now youre number 110 in line, and you DONT get into the next pre-show, delaying you by however long an additional ride cycle takes.
Bathroom, okay. Drink? No. Cigarette? No. Neither of these is generally an emergency, so if one or the other or a FastPass is so important to the Guest that they need to leave the line, no, neither is a valid reason to catch up with the rest of their party.
Somebody a few pages back posted a "rule" which, while not a rule, makes perfect sense: If a member or two of your party must exit the line for an emergency, the rest of the party should reasonably remain at that point - letting other Guests pass them - until the one or two get back.