Good point as often a stock will rise once he gives his backing...

Good point as often a stock will rise once he gives his backing then he may be selling out before publicly telling people he has changed an opinion, easy way to outperform the market.

Motley Fool a classic example as they suggest a stock in their subscription service and usually runs 10% the next week, they then run 5 articles a day about it and ramp the bejesus out of it. When stock advisers get big enough they can move markets, even worse on top end of town when JBWere builds a position then upgrades on a stock for example. Or could short IO a day before they plan to releases a massive negative outlook (6 months ago say..)