It is NOT a core feature of a currency, in fact although some jurisdictions might not currently prosecute people for writing messages on e.g. dollar bills, I believe there was a time and/or are or have been jurisdictions where "defacing a coin of the realm" was not only offensive but "an offense".
Is Bitcoin supposed to be only a currency, not a payment system?
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Bitcoin is a currency. Stand by for a payment system, "coming soon". (4 to 6 weeks or so? 
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)You mean Ripple? I'm very enthusiastic about that, but I'd also like to see basic messaging functionality in Bitcoin clients, including the reference client.