I main RDM, I play DRK, WHM, BLU, SCH, and the occasional other job, because RDM is wanted for nearly nothing. If I am forced to pick certain jobs to goto a higher level, it would likely come to this list of priority for me right now.
1) DRK
2) SCH
3) RDM

I main RDM, why is my list like this? Because these are the jobs I get to play in events. As much as I love my RDM, I would have to pick it 3rd because unless my DRK is going to events, my RDM is getting no new gear, no new things to make it grow, it is worthless to me at lv100.

"You get invite on your best geared/favorite job" Since when, I would get invited as RDM to endgame as much then, as I do now, oh wait, thats almost never! Making specialized things for jobs as you suggest is just another way to make people put more into jobs they rather not, instead of jobs they want to.

Want an Example we already have in place? Merit WSs, I want Upheaval, I like WAR, I have been gearing it better, I don't always want to have to use Raging Rush. At the same time, I main RDM, and made Excalibur, Requiescat is a near must for me, without it my Relic would lose alot of its use when you think about me having an Almace as well. Resolution is another must have WS, why? Because its that strong, I geared up my DRK alot, I can tell you for sure, if I were to goto a VW party & start using something besides Resolution(mind you, no Emp/Relic) I would be kicked instantly. I also play SAM, but have no Masa, guess what that means? In these things I have had to give up having fun on some jobs I like in order to have efficiency, and power, on jobs I need to go for events, limiting the power of my jobs I like, and instead forcing me to put power into jobs I either do not like, or do not care to "specialize" in.