Wow, that first day hit to your new MMO’s servers is going to suck, isn’t it? It will be the busiest time your game has ever had, and it will be the first time you see all its problems. Thousands of people will be hitting your account creation, billing, download, patching, and log-in systems at the same time, and thousands are already prepped to go complain about how slow/crowded/whatever these things are.
Enter head starts. People will pay you a bit more for your game in advance, or they will pre-order, or both. Open the server a couple days early and let them give it a dry run. If the account creation system has problems, oh well, you already know these are paying customers. You’ll get it fixed by the time the masses arrive. When those masses arrive, a first lump of players has already moved past the very beginning content, spreading server load and reducing crowding in the newbie areas. These are probably the hardcore players anyway, so get them in a little early to start setting up guilds and the player-based in-game infrastructure your game expects. You have spread out account creation, downloads, patching, log-in, and early content, and you got people to pay you more/sooner for the honor of helping you. Good call!