The premium is so cheap because any expense below 8L has to be borne by your corporate's insurance, anything above it will be paid by the insurance company. The problem is if you are out of a job, you have to pay anything less than 8L (might be way more less but depends on policy to policy) from your pocket
It's like co-payment and aggregate-deductable. Correct me if I am wrong.
It's better you go for a separate health insurance for yourself & family rather than this top up so that if tomorrow if you are out of job, you are still 100% insured/covered. Also when you are in a job, you can claim from your corporate and your personal insurance company in the worst case scenario.