Two comments:

1) At YUL, it has happened to me a few times that I waited in the J/E line, while a steady stream of SEs used not only their dedicated agent, but were also waved ahead of everyone else in the ever-growing J/E line. This completely defeats the purpose of having a J/E line in the first place.

2) At YVR last Monday, at around 0715, there were only two agents total working J/E and SE. It took a few minutes until they realized what was going on, and then two more agents joined in to handle the J/E line. By this point, there were 20 people waiting.

These two example should, IMHO, never happen. The whole point of the supposed benefit of the separate check-in is speed, which did not work out well in these cases.