I think casinos at designated areas outside of the main cities may be ok. Maybe even creating a special zone as a gambling and entertainment area. But placing it in Bangkok, Pattaya, Phuket, etc will only serve to increase the wrong kind of tourists, traffic, rents, and basically make things worse for anyone actually living there Thais and expats alike. Tourist areas are already too packed - wouldn't hurt to create a new one specifically for that.
The issues of gambling accesibility and addiction are moot because Thais already have ample local access to illegal gambling operations. If anything making it legal and administered by established large scale operators like MGM will probably decrease illegal activities while making profits taxable. As long as they keep operations clean and avoid corruption and bribery. This being Thailand....I'm not so optimistic about that.
P.S. Online gambling should be legalized. No reason not to.