Except these are not casual projects I agree with your attitude here but not all of the context, if your pugging often then its a little more daunting over time if that makes any sense. It has nothing to do with someone on occasion soloing in the team it happens in pugs a very high percentage of the time, so its understandable when you know you could be getting the mission done allot more efficently and allot more fun let me give you an example. If I am playing a kinetics buffer/support then if people are split up, no one else but the group im with will benefit from fuclom shift, end returns, as well as heals, and the other side are dying often this is not fun most of the time for others in my experience.
This is not that kind of mmo where you can solo doing other things unless it is an easy mission, teaming is not about just joining a team its about cooperation, and this mmo is a heavy cooperation based mmo much like other older mmos at the time, its deep in the mechanics and you cannot really change that without breaking the game. Soloing is for soloing that is why it is called soloing lol.