I've got three taps so that I can have enough variety on the go. Stout, IPA, Pilsener. Chilling another keg may be an overnight job if you start from room temperature, but if you have a fermenting/lagering fridge, you can have it pre-chilled or at least cooler than room temp.
The amount of horizontal space in a KK4 is nice. Big enough for three kegs and a few jars of yeast or bottles of beer. The amount of vertical space above the kegs is very minimal though - that makes it hard to attach and remove the disconnects once a keg is in place.
I have also put the manifold on the outside and just removed the plug for the gas hose. That makes it very easy to run three gas lines into the fridge and operating the manifold from the outside is much easier. You can do it without opening the fridge and letting the cold air out.