I don't know how or why it actually works, but on the Red, the blue channel is already a little noisy. Balanced under tungsten, it's more so. The 85 actually reduces the noise in the blue channel a lot.
Thing is, for students, tungsten is usually whats around, not to mention cheap compared to most Kino's and HMI's. Adding an 85 to help with tungsten is a much more cost effective fix for those in this type of situation.