First of all let me mention that I am a dentist. Regular professional cleaning (Scaling) once in 6 months is recommended. I would say if the oral hygiene of the patient is good, once in a year would be sufficient. If done in the proper manner by a professional, it is not at all harmful.

It doesn't "weaken the roots" as ur friends say. It doesn't reduce the "lifespan" of the tooth as well. Scaling has additional benefits over regular brushing because "Calculus" (or yellow deposits or tartar) will build up over time at inaccessible areas and will cause bleeding gums initially which is due to the gum disease Gingivitis, which will progress to the gum disease (Periodontitis) over time. Periodontitis, left untreated will cause bone loss around the teeth and make the teeth loose. This is a very slow process (takes many years) and is not painful, therefore the patient will not be aware of such a condition.

That is the progression of the disease, of course the amount of calculus and gum disease..varies from person to person. Some people have more calculus, therefore, more gum disease...while some people have less...

But, whitening is a completely different process. It is a chemical process using strong chemicals (hydrogen peroxide) activated by strong light. Repeating the whitening process regularly is not advised, as it would make the teeth sensitive(to hot and cold) or in the worst case scenario cause pain which will be relieved only by performing root canal treatment. Whitening should be done by people who have the will to maintain their teeth in that condition by reducing or avoiding the use of tobacco, strong coffee (or any colored drinks). They should maintain good oral hygiene as well.

I am not saying that if you get ur teeth whitened u r going to have problems, but repeating the process again and again in short intervals is not at all good for ur teeth. Hope this answers the question.