You don't have to, this is why I like steam.
The only exception to the rule of having to buy separate version of a game on steam is Black ops.
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The point was to have a Linux version. Way to go, The Verge.
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The listing still says Windows, but it's been confirmed!
http://www.theverge.com/2013/1/25/3915338/original-half-life-finally-available-for-os-x-through-steam
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