Hold kick when when you can get 2nd part to hit. When far out (meaty d.mk, d.lk, qcf+k combo) I don’t see the point because you’re just leaving yourself open.
Right-up-their-ass HCF+K (old school s.RH) can crossup certain characters (Shotos), and makes you switches sides. If you hold K, the overhead part will stick you even further out past them (which is why it will avoid wakeup DPs and flashkicks and stuff), but atfer the moves is done you will go back to the original side (if they are certain character, or don’t attack, blah). It’s all terribly character speicifc, and some even only in the corner.
It’s also really buff in combos because of how it rocks the opponent across the screen. Reminds me of Mortal Kombat. 
EDIT: mistakes…