During my reading of Dan Anderson's book on Modern Arnis he refers to Tapi Tapi as a drill I (was told it was called) Block Check and Counter and Obstruction Removal drill.
Dan calls it Left on Right Single Cane Sparring and Right on Right Single Cane Sparring. I learned these drills as Tapi Tapi left vs right and TT right vs right.
These drills the single cane sparring drills are the ones that I meant.
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.