Except that White was already in year 3 and came in somewhat more advanced in terms of his collegiate experience. I don't know that there is a whole lot more that White was going to show you that you haven't already seen. I'd have been fine with keeping him, but it's clear the coaches were not comfortable with any of the trio of Montgomery, Janis or White being the regular #3 guy, hence the Jones signing. The Packers play with 3xWR so much that is essentially their base offense and the #3 guy is a starter.
I think the position Janis will be in next year is the same one White was in this year. He really has to stand out somewhat and earn a roster spot, but I agree they kept Janis based more on potential (and Montgomery because he was a higher draft pick and more important on special teams). If the potential doesn't come through as production next camp then I suspect a rookie will grab his roster spot.