Why is the paddle and not the whisk used for making cake batter in a planetary mixer?
In the recipe book for my Kenwood (I'll try to find it to post examples), there are some whisked cakes, but most are beaten. This is a UK model, and we tend to use solid fat rather than oil in our cakes. The whisk tends to collect the fat inside, rather than mixing evenly, so you need to knock it down more often. With unsoftened butter the whisk also flexes quite a lot which may not be a good thing long term.  When mixing slowly the beater definitely mixes more evenly than the whisk (a reasonable approximation of folding at minimum speed). 

Whey baking other recipes I use the whisk for a whisked recipe (usually folding the flour in by hand), and the beater otherwise. It is quite easy to over-mix though.