There is a third option which you have forgotten and in Klitchko's case makes it imperative that he clinch. It is known as the glass jaw. His chin is so vulnerable that he can't risk being hit. The safest way to avoid being hit is to adopt a John Ruiz style of fighting were less shots are thrown and only quick jabs get out followed by a clinch. Because he's huge his jabs hurt more than someone of normal stature would.Based on my experience, I find that a clinch (hold on to the other fighter) is for fighters who are either tired or hurt.
The fighter that has the stamina to go the distance and hasn't been hurt has no need for clinches.