A lot of the recipes are old and were written for people mixing the batters by hand. With the new powerful mixers, there is no need to "add eggs one at a time until well mixed". Other than the creaming part (not even that step sometimes) and beating in the eggs, I add everything else all at once and have been doing so for a very long time.
There is one particular cake recipe that my mother would make by adding all the ingredients other than the topping ingredients into a pan and beat it up with her hand mixer. No one ever complained about how that cake would come out.