Here's what the official GHD site says under FAQS about using heat protector if you straighten your hair everyday using their iron:
“I would recommend using one of the 3 types of ghd thermal protector. They all contain UVA protection and the thermal protector for normal/fine hair and dry/coarse hair contains meadow foam seed oil, which maintains the moisture content, and helps increase the elasticity of the hair during heated styling process. They all help to deflect heat away from the hair"
If you don't straighten it everyday, then I don't think it's that big of a deal to skip the protectant. But it's important to remember that no matter how great a product is, if it's applying heat to your hair, it's damaging it. Also, damage isn't evident without prolonged use sometimes, so better safe than sorry IMO.