Answer: Nowadays Online Trading is very common so, one should have to give some kind of authority to his broker to trade easily and faster. The whole criteria of POA revolves around the customer will now. It quiet safe now because of SEBI rules and regulations.
"According to the SEBI Circular dated April 23rd 2010 and aftermath clarification about few doubts on the said circular dated Dec 30th 2010 indicates that POA is mandatory only for online accounts but not to offline accounts."
One should issue specific and revocable POA to his broker for his protection and safety, because its rightly said precaution is better than cure.
POA is applicable only to Demat Account and on other equity and future option its optional not compulsory ,its the sole discretion of customer not the broker.
Nobody can sell shares without consent of customer if you didn't give POA to Demat Account issuing company one should give Delivery instruction slip(DIS) every time you sell shares to transfer it to any other company to the broker but take huge time , so its better to issue Specific POA with clause and revocable .
One should go through this article for better understanding.
http://articles.economictimes.indiatimes.com/2011-07-11/news/29761256_1_stock-broker-poa-transaction