Having waited a couple thousand tables at a couple thousand brunches and buffets I can say honestly we all should get a new plate every time we go up to the buffet service tables for cleanliness sake.

While you were using the plate you started with you did transfer a small amount of bacteria and germs to the plate and if you go up and use the public utensils in or on the buffet to serve yourself again you might touch the utensil to the plate and cross contaminate the utensil transferring your germs and /or bacteria to the public use utensil.

Any restaurant owner will tell you or anyone this because it is by FAR cheaper to wash more dishes than fight the negative press associated with patrons/guests attributing the food borne sickness they just came down with to the restaurant they just had the buffet at. One big or even small story that reflects poorly on the health and sanitation of a restaurant can dramatically lessen business and/or close the doors forever.

Always lean to the side of food safety and cleanliness. New plates every time.

Whether you consider getting a new plate to be an economic, hygiene, courtesy or moral issue, the resounding answer is: YES, without doubt get a new plate.

Economically speaking, clean plate users are helping employ a busboy or server, and a dishwasher. They are also helping dishwasher soap manufacturer employees, city sanitation workers, plate manufacturing employees and on and on.

Hygiene speaking: Food borne illnesses can be severe to the point of deadly. (See the Center for Infectious Diseases link listed below.) Approaching a buffet with a used plate can cause contamination in a multitude of ways including a serving utensil touching the contaminated plate, droplets of old food or contaminants stuck to the bottom of the plate from the eating table falling into the buffet, and overflow droppings of new food hitting the plate and then falling back into the buffet when being scooped.

Courtesy dictates that the consideration of others comes before our own. If it were you, would you want to take mash potatoes from the buffet after the person in front of you put a scoop on their dirty dish and used the serving spoon to swirl them around making a nice well for the gravy?

Morally speaking, one might be carrying germs they are not yet aware of. In a public space, and especially when partaking of "communion" (shared food) with others, they want to take utmost precaution to keep it safe and pleasant.

Health departments of every state have issued many, many regulations for safe food handling. (See below.) As an outreach of the general public, Health Departments evolved to having field inspectors, dedicated websites, research funding and on and on. This is a direct result of the general public recognizing the severe impact of food borne illness. Here is a quote from the Louisiana State health Code that typifies the regulations of all States: "Buffet lines must be adequately supplied with enough clean tableware, so that the customer may obtain clean tableware for each trip back to the buffet. A sign must be placed at the buffet prohibiting the reuse of soiled tableware."

Although I agree with @buddawiggi it is up to you...
I personally get a new plate...I hate when my goods mix...grosses me out...
Some people feel bad piling up dishes but honestly...they hire people specifically for collecting and washing dishes...so it's not a big deal to get another plate.
So I would say use a new plate every time...not only for health reasons but to keep your food separate....