People need to stop saying this. There are a few brands of Laptop (Dells Alienware 17 and 18 series for example, Sagat as another example) that have both HDMI output and Input.
http/www.dell.com/ca/p/alienware-17/pd.aspx
That link, you can read the technical specs on the 17 series laptops and they confirm Input. I also own a 17 series laptop and use the input to play on my PS4 when my girlfriend uses my large monitor to watch television.
MOST laptops yes are output only. but there are a few brands with input. DO your research people.