Reasons:
1) the game may have folded/collapsed by then and exposure to the richest gameplay period may be missed.
2) the game may fold if there aren't enough initial bandwagonners. Not enough initial revenue may also lead to smaller roll outs of content. this is less likely on a AAA game but hell even the Old Republic didn't fair well and that MMO exceeded guildwars and WoW in gameplay and experience (not endgame retention vs WoW though).
3) If you have friends playing months ahead of you, you will be behind for a while, which isn't a huge problem. I personally enjoyed doing a bunch of questing with friends in MMO style games. Of course you will advance through content potentially much faster in a good group.