To keep the game interesting and more replayable I think that over time new races and classes should be added over time. With only 3 classes and 3 races at the moment the replay value would probably be less if these were the only ones throughout the game's lifespan. With new races and new classes this would add a greater in-game diversity and keep gameplay fresh for both the campaign missions and the Crucible.
I do realise that Bungie may add more races and classes over the game's lifespan but I'm just voicing my concern to the possibility that they may not.
I've added a poll to see how much or how little people agree with this.
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I don't think it would be good because we'll be spending all this time upgrading and levelling up our characters and then when a new class comes out that we prefer then we'll really want to play as that character but we'll have to start all over again!