what is the purpose of adding oil to balsamic vinaigrette slowly? I've been making a lot of vinaigrette dressing lately. Julia Child and Emeril both say that you need to mix up everything but the olive oil, then add the olive oil in slowly (ideally drop by drop) while whisking. 

First, this is a pain because you need 3 hands. Ok you don't really, but it isn't easy to do with 2 hands. 

Second, it seems like I still need to shake the dressing a lot before putting it over salad, especially if the dressing has been in the refrigerator. 

Finally, the question: what is the upside of all this doing-it-slowly when you add the oil to the mix? As far as I can tell, it makes a tiny tiny bit of difference: there will still be some tiny tiny drops of vinegar suspended in the oil a few days later. Why would Julia and Emeril think this is such a big deal?
Specifically, the reason you whisk it in slowly at first is to create small drops. If you just dump the oil in fast, it will adhere to itself and make it impossible to break up into droplets dispersed in the vinegar (or other water based liquid, such as lemon juice). This is the definition of an emulsion: tiny droplets of one liquid evenly dispersed in another. Once you've got the emulsion started, you can pour in the oil a bit faster because the already suspended droplets reduce the room for a big blob of oil to form, and act sort of as "sandpaper" to break up the incoming stream as you whisk.