I completely agree that using exchanges as your wallet is an improper way to use Bitcoin, but using them to buy or sell is perfectly acceptable.
Yes, after all, it's not realistic to believe people will only get Bitcoin by mining it or by asking a friend to share some for free/actual goods. You're mentioning using exchanges as wallets, but I was also thinking about wallets in general for a while. We all use them, but it means that we are often again trusting a third party to keep our coins safe (okay, sometimes there's no way this third party can break into our accounts, but it still exists as an intermediary). So basically, unless it's a paper wallet kind of situation, even if one is not using exchanges, one is usually not using Bitcoin the way Satoshi had in mind.