No matter what the developers create a quick and efficient strategy will soon be developed. People who want to clear the content quickly and efficiently will use these strategies. If you want to complete content with a less than optimal set-up then you are free to set up your own runs. People who use these strategies are not sheep, they are some of the few capable of researching a fight and getting a group together for it. Personally if I'm farming seals with my Linkshell then a sub-optimum set-up is fine. I'm helping a friend and we're hanging out in between pops, chatting etc. If I'm doing seals with a pick up group of people I don't know I definitely am more inclined to just get it over with. The fastest way to get it over with is bringing the optimal group.

Voidwatch is by far the most job diversity friendly event in the history of the game, if you think otherwise then you don't really know what you're talking about. Yes, there are "R/M/E only" shouts but they are by no means the only shouts out there. I am often able to join those groups, on my Dragoon, with no R/M/E. Will every job be useful for every event? Of course not,that's a pipe dream and to think otherwise is beyond foolish.

What it comes down to is even if any job can contribute to an event there will often be a job that can contribute more, which speeds up completion, which means people are finished faster. They can then tackle the content again for another chance at rewards or move on with there day. Personally I'd almost always rather get more done for a chance to better my character than get less done and not advance my personal or (in the case of my Linknshell) group goals.