I dont short and never have but I can see opportunities more often on the downside than on the long side.
Keep in mind for every buyer you have a seller( if shorting wasnt allowed) and you wanted to buy shares of NOT who would sell to you? what if you wanted to buy 50,000 or 100,000 shares ?
You need liquidity in the stock...people have to play both sides otherwise there would hardly be any volume cuz people would just hold and wait.