This is myth that cleaning weakens your teeth. We all require a cleaning session atleast once a year to maintain gums and clean deposits from area which we are unable to clean ourselves with regular brushing. Cleaning/Scaling is a prophylactic treatment. It does not remove the tooth layer but the plaque and calculus from your tooth. During scaling, there may be mild bleeding, it is not due to any harm to your oral health but due to the infection in your gums and bones. You may have slight sensitivity after the scaling procedure, it is because earlier you were carrying chunks of calculus over your teeth and after it is removed your teeth is exposed to the oral fluids again. If you had grade 3 periodontitis, after scaling you may feel slight mobility in your tooth. It is not because of scaling. It is actually because calculus causes bone loss and thus reduced tooth support and calculus(dirt) was holding your tooth. Now once that calculus is removed, you may feel mobility of tooth, but it will heal now. If you do not remove that calculus, it may eat the complete layer holding the tooth and the tooth will shed off automatically. Apart from that, calculus will also spread to the other teeth. All these symptoms will gradually go off within 7-15 days of the treatment.