It is acknowledged that Alcohol and Cigarettes damage health and wellbeing, why add to it.

You mean why make things worse by making Alcohol and Cigarettes illegal? I agree. It wouldn't significantly reduce the damage they do, and at the same time prohibition would have terrible secondary effects on society,

And by the same token, decriminalising cannabis use would be unlikely to increase significantly whatever harm it does, and would reduce the harmful secondary effects of prohibition.

I'd favour "decrinminalising" rather than legalising, so that possession for personal use, growing your own, and supplying on a small scale would be free from criminal sanction, but attempts to make a tobacco style cannabis industry would be severely punished. Perhaps some kind of licensing scheme for suppliers might be the way to work that one out, with no licences for the big guys and the big companies. Or for anyone with any record selling heroin and so forth.