We can answer this question, but first we must make it operational. Let's measure a "usage" of bitcoin as a bitcoin transaction. This is the most simple, general measure of bitcoin usage. Now we have a quantity. We can graph it. [1]

Now, we just need to get some data on whether these transactions occurred due to commerce in bitcoin, or due to trading for a profit. We can't see this on the blockchain directly; however payment processors like Bitpay and Coinbase have access to this information. Coinbase, in particular, operates both an exchange and payment processor. It knows what percentage of transactions (within its market segment) are due to buys and sells vs. ordinary commerce.

Luckily, Coinbase has gone on the record answering this specific question! [2] Brian Armstrong, the CEO, said specifically at Techcrunch that the majority of bitcoin uses are for buying and selling things -- actual payments. [2]

So the answer to your question surprises you -- the majority of usage is very currency-like!

[1] https://blockchain.info/charts/n-transactions?timespan=all&s...

[2] http://techcrunch.com/2014/10/21/coinbase-ceo-brian-armstron...