They're not pointless, they're just scaled down to simple support units. The speed advantage of the Whites and the strength of the Purples are very much still a part of your strategy options.
Nonetheless, it still baffles me that the Red, Blue, & Yellow Pikmin-types had received upgrades to make them more flexible, but both the White & Purple Pikmin-types have been reduced to, as you've fittingly put, supporting roles. If both types were over-powered in the previous game, wouldn't it be a lot more simpler to just create foes that can counteract some of their strengths?
However, that's a minor complaint, and in the chance that we do take control of Captain Olimar & Louie to retrieve more treasure (and such) in the future, I just want my boys in White & Purple to be back in working order.