Theres alot of people encouraging new companies to use Bitpay or Coinbase as merchant solutions, just so they can accept the currency and they can take no risk. The problem with this is that it drives the price down.
Users who pay with bitcoin, have had that bitcoin, meaning they didn't just buy it to spend. Only a few are buying bitcoin just to immediately spend it.
For the people who already have bitcoin, when they spend it at these merchants who are using Bitpay or Coinbase, and settle in USD, they are just driving the price down.
Overstock.com - They accept Bitcoin, they probably did so many transactions today, but all of those coins were dumped on the market. Thus driving the price down. Imagine how many bitcoins they made today, but they didn't hold onto any of them, instead they just dumped all those coins. Anyone who paid in Bitcoin on Overstock today, pretty much just helped lower the price.
The fiat money, which overstock actually accepts, has to come from somewhere. This means someone has to go and buy those BTC used to purchase stuff from overstock. So you have an ongoing "circulation" and people buying BTC signifies a demand for it, which means a higher value for BTC, which means "cheaper" (relatively speaking) items on overstock, which means people might be motivated to buy more BTC,...etc
Which also means nothing happens without using fiat money.
And in case people do not buy BTC the price would ofc go down or in case large quantities of BTC are used to "play" with the market. But this second possibility is more of a problem when it comes to top 10 BTC holders.