A because it actually gives me a clear direction. If I got into a game and there was no immediate task, I would wander about clicking on things until some objective made itself known. In your B scenario there is no objective. You just hope players would feel compelled to buy a piece of gear, realise they need a dropped currency, and then go out and grind it without being asked to. The thing is, that without an objective you are going to confuse people. I might not know where the mobs are, I might miss the vendor, I might think "I don't need/want that piece of armor because it's ugly/has bad stats for me/reasons" and decide the grind isn't worth it. What then? I just log out because I have no direction and don't know what I'm supposed to be doing if not just grinding mobs to buy pieces of armor.