That's true, why not just fill it up?
Headspace shouldn't be bad for primary though (to answer you seriously), because you'll be pushing out a lot of CO2 to form a cushion. However, I'd want to pull it out and into a carboy if you dry hop or bulk age.
I top off my bulk aged beers to the very top of the carboy to make sure no O2 gets in.
Primary: Hefeweizen, Scottish 60-/, Lemon Wheat, Imperial IPA, Apfelwein
Secondary: Sweet Mead, Dry Mead, Oaked Dry Mead
Bottled: Sunbreak Amber, Irish Coffee Stout, Bavarian Weizen, Bigfoot Barleywine Clone, Imperial Saison, EKG Lite, Dry Mead, Spiced Apfelwein
Kegged: Strawberry Blonde, Ballard Bitter Clone, Southern Cross IPA, Smoked Habenero Porter, My oh Maibock, Maris Otter Mild, Apfelwein x2