I will agree that Tapi Tapi is a drill, because it has a number of constraints (both explicit and implied). I will base my opinion most largely on the lack of distancing involved in its practice (as done by most--I do not mean to generalize everyone's methods into one statement.)
'Sparring' is a term that should be limited to exchanges with as few constraints as possible.
If you are sparring, it should look more like Dogbrothers, even if it is done lightly. Isolating certain skills should be considered a drill (for example, sparring in which the hand is the only target, for example). I like isolation drills too--I think they are awesome. Especially alive drills that are unchoreographed, and involve improvised footwork and timing. I am slowly building a list of fun ones.
~TT