Funimation has this nasty habit of, any time they do a DVD release of anything Dragonball-related, always leaving themselves an out so that they can rerelease it later and bill the new release as the "ultimate, definitive" version.
The original Z singles release were censored. Then they released the "ultimate uncut" singles, only they never finished the series. Then they released the orange bricks and, well, don't even get me started. Now we have the Dragon Box Z, which is ALMOST perfect, unless you like the Faulconer score. Or want it in HD. Or want to wait for Kai.
Given that Funi's goal is obviously to perpetually have a Dragon Ball / Z DVD release on the market at all times, there's no point in my waiting for the perfect release (or even the "as good as we're ever going to get" release), because it will never come: there will always be something around the corner that's just a little bit better. Instead, my strategy is to wait for the release that's good enough that I won't be tempted to double-dip when they inevitably release a better version a couple years later.
In the case of the Dragon Box Z, yeah, I could wait on the inevitable HD Blu-Ray release or Kai, but neither of those tempts me so much as to make me wait, so I'm buying up the Dragon Box DVRAB and calling it good. I have absolutely no doubt that there'll be another release announced right after they finish the current release, but none of the potential selling points of such a release would be enough of an improvement to make me want to double-dip.
But back on-topic, as far as Dragon Ball goes, if you're already buying the blue bricks, a Dragon Ball Box probably wouldn't be worth a double-dip to you. Personally, I'm confident that a DB Box is just around the corner once the blue bricks are done, I enjoy the Japanese version of the show far more than the Americanized version, I want a version that matches my Z sets, and I only want to buy it once, so I'm renting the blue brick versions through Netflix and waiting on a Dragon Box release before I buy.
I certainly wouldn't blame anyone who went the other way, since the blue bricks are almost certain to be cheaper and they're not hypothetical, they're coming out NOW. It's a personal preference thing.
When the Piccolo stuff starts coming out I may take the plunge and buy the blue bricks for that, since that's one of my favorite arcs in the entire DB franchise and I wouldn't mind double-dipping if/when a Dragon Box release came out.