This is a good question, and one I've asked myself a lot. I'm of the opinion that as long as the alternator has the necessary capacity high priced and/or add'l batteries aren't that beneficial. I think a good quality/healthy regular lead acid battery will do 99% of what an expensive one will do.(On the other hand, if the alt is under-sized, a big battery might cover up the day-to-day issues, but the alt's going to fail eventually, then the quality of the battery won't really matter -- it's dying regardless).
I believe the mfg's claim their batteries are lower resistance than a typical battery - requiring a bit less energy from the alt to maintain their charge.
I'm guessing they also have a little more available power in that 14+ to 12.6v (float charge) range.
Maybe a $300 battery believer can shed some light on this.