Line cutting can be a very sore subject on these boards.
In general, most people here (myself included) tend to feel that cutting into a line that you were not previously in is poor etiquette. If you want to be seated with the rest of your party, then the entire party should wait and enter the line together. Otherwise, the fast pass runner should use the single rider line (where available)--or skip that ride entirely--and meet back up with your party after you exit.
This is a different situation than someone who enters the line with their party, then needs to exit--to take a young child to the restroom, for example. I have no issue with allowing them to re-enter the line to rejoin their party. Key difference being that 1) they did enter the line with their party originally, and 2) leaving the line temporarily was due to a genuine need, and not something done for mere convenience (like running for fast passes).
My family does use the 'fastpass runner' approach at times. It is faster than dragging everyone back and forth across the parks, just to grab the next fastpass. But we always either wait by the entrance to the queue, or go do something else, until the runner returns. Then we enter the line together for those rides/attractions we wish to experience together. Works well for us, and we don't have to worry about ruffling any feathers by attempting to cut in line.
