In my opinion, pump & dump groups and the activity itself serves only the creator and close circle around him/her. So far, many people lost funds due to it and should be kept out of the industry. There are many other legitimate strategies around to be used instead of it (as to not spam with long text, few can be found here:
https://www.cryptocointrade.com/about-trading/crypto-trading-strategies/)
On "bitcoin" there is really no one and no group that is really powerful enough to pump and dump. However on smaller altcoins they can actually increase and decease the price of a coin very quickly. There are really two possibilities for a pump and dump afterwards, one is that coin actually going up and when goes down it is still more than what it used to be which is a great scenario for the owners of that coin.
However, the bad part is sometimes it goes up but goes down harder and goes even lower than what it used to be. Now when a pump and dump happens it makes the coin heard, if you see a coin that goes up 50%+ in a day you learn the name and they may get couple more investors thanks to the awareness created by the pump and dump group and if the coin doesn't go below what it used to be that is the best thing that could happen since its a win-win situation.