As a new player myself, I've chosen to wait to purchase HW. As far as I'm aware, there's just no reason you have to purchase it until you have completed the main storyline quests up to and including the patch 2.55 content at 50 unless you want to play the new race OR one of the new classes (but there are requirements to be met before you can play one of the new classes anyway, and you can't do those until you're level 30 I believe).

Here's a thread about what must be completed before you can go to the new content:

http://forum.square-enix.com/ffxiv/t...-now-you-re-50...

Much of the content that must be done at 50 requires groups to do it, as well as a minimum gear level. Some of the later story quests now reward gear to help with that part, but you still have to get a group to do it. Groups seem plentiful ATM, but how long that will last is anyone's guess. That's the part that prompted me to wait to purchase, since if getting a group for the required content is difficult, that could put a halt on getting to the new content at all.

Hope that helps. :-)