this is a very good question. in my opinion, white chocolate is a very fine source of sugar and butter, plus emulsifiers. in a buttercream, replacing part of the butter with the white chocolate cocoa butter adds room temperature stability since cocoa butter has a higher melting point. cocoa butter is also more delicious than butter.
adding white chocolate to the mousseline should be done not in excess, as white chocolate is very sweet.
some good examples of using white chocolate instead of the more traditional butter and confectioners sugar are on the white chocolate cream cheese buttercream recipe on Cake Bible and on the Creme Ivoire.
other applications of white chocolate are on cake, like on the Golden Luxury Butter Cake, where the white chocolate provides the sugar, the butter, and the emulsifiers present on most all chocolate make the texture of the cake dissolve in your mouth!
btw, i keep my white chocolate in the freezer! and for years. it is a good way to always have some white chocolate on hand, as it is usually not that easy to find (most supermarket varieties don’t contain cocoa butter). vacuum pack, and be sure to let it reach room temperature before you open the vacuum bag. you don’t need to thaw it first in your fridge, provider your vacuum seal is good.