I like buying and selling. I'm not always the best at it but it keeps me entertained. Unless there is some kind of market there is not much incentive to keep a block chain going.
Sometimes I like to think about what would happen if everyone acted in a certain way. If everyone just bought and held there would be no sellers. Without the potential for movement and exchange a price is hard to find.
In a way selling your cryptocoins is some other person's ticket to get into the market. In my opinion playing well with other fiat currencies through trading is a good way of improving many lives around the globe. To me, it is much better than hording it unless the price is really going down and your a whale dumping on the market. It is also better than cashing out and buying up precious metals or other scarce resources for hording.
If your trading Steem for BTC either way what your holding is valuable so you might as well do some trading if you ask me. Sometimes cashing out helps pay the bills too but I would retain enough so that your not kicking yourself later for not holding on to more.
I do agree, trading increases liquidity, but there's quite a bit of that now. I'd like to see more liquidity at Open Ledger, so I do try to buy/sell there when I can.