Time is rather tight right now, so I apologize for not reading this whole thread yet.
I hear of a lot of people buying multiple copies of the 55" rails for joining them together. In my opinion, it is a waste to have more than one of the same size rail (unless storage/transportation is the reason). If you can't have the 9' rail, then you are better off having a 55" and 75" pair instead. This gives you more flexibility than duplicate 55's when they are used separately.
Aside from the storage and transportation issues, I personally prefer having the wide array of rail sizes at my disposal. I even have the little 32" rail, which I love! For my tastes, I prefer having the ability to just grab the rail length I need, rather than messing around with joining them. On the other hand, if I was transporting them to project sites, I do at least have the joining kit so I can still use a combination of my shorter rails if necessary.
If you have the ability to store the 9' rail, then I believe it is well worth the investment.