Lots of good reasons, for us we like to shower in our rig. It has a nice shower and the 6 gallon water heater is more than enough for two decent showers. First of all you are mixing cold with very hot to get nice and hot, around 101 degrees works well. The moment you turn on the hot water to start your shower the water heater fires off and starts replacing the heated water you are drawing off. It keeps running while you dry off and step aside fort the next shower.
There is a lot more LP burner under that little 6 gallon tank than most folks think and I have never seen it take more than about 10 to 15 minutes to go from cold to very hot. If you can't get two 5 minute showers out of your 6 gallon tank something is seriously wrong. Having said that, if the CG showers are nice and clean, convenient, and heated is an additional bonus in the colder months then sure, why not use the CG showers, you're paying for them whether or not you use them or not. During the warm summer months we frequently use our outdoor shower, and yes we wear our bathing suits. :B This dramatically reduces the amount of water going into the grey tank.
As far as the restrooms same story. If they are clean and convenient that's where we go for the morning sit down. If not our toilet works just fine. I am always amazed at how many people insist they have to have full hook ups. If you are going to be there a week I understand but 2 showers a day and normal use of the toilet combined with a stop at the dump station on the way out has always worked very well for us for 3 to 5 days and we are rarely in any one CG longer than that.
As always.... Opinions and YMMV. :C