it's a lot easier to stay within one brand that has designed step downs with different balls and ball lines
Sure, if you buy all of your equipment at the same time, and do not intend to replace it any time soon. But, if you are looking to update your equipment year to year, there is no guarantee the latest, greatest ball from Brand X is going to be an improvement, or fit neatly into your arsenal.
For example, a few years ago I was rolling primarily Roto Grip equipment. I thought the Cell Pearl was the best ball ever, and naturally expected Roto to continue manufacturing it, or make a better ball. Instead, they made a different ball, perhaps better for some, but not for me. I tried a few other balls in their new lineup, with less and less success, and gradually became less enthusiastic about the brand. Ever wonder what happened to Wes Malott after his tremendous run the year he was rolling the Cell Pearl? Check out his career statistics after 2008. See http://en.wikipedia.org/wiki/Wes_Malott.
I have also tried Visionary equipment (even became a test staffer), and liked a few of their pieces, but not every ball in their lineup was a star performer, either. I had a nice run last year rolling predominantly Hammer equipment. But, it had more to do with one ball, the Infection, rather than the entire brand. That ball is no longer available this year, and unfortunately, I could not find an equivalent replacement in their new lineup.
So, what's a bowler to do? I say invest a few dollars in research and get the best equipment for your game, regardless of brand.