My 2006 has the white roof also. I too believe is is to reduce heat to the interior. The sun causes the paint to oxidize and it forms a dust on the surface. (My 1991 Camry does this too.
) Rain / dew mixes with it and that causes the streaks.
I used the Walbernize wax on the top as well as the sides, this slows the paint oxidation and eliminates the 'milk run' down the sides. It works on the Camry too (Turtle Wax Hard Shell wax) but it has to be done a couple of times a year. I choose late spring (April) and early fall (October). Temps are lower and I enjoy the time outside more.