Is TA-ing worth the opportunity cost (of having more time for research)?
Most people who TA don't have an option as it is what pays the bills and allows them to study. It is less valuable if you can pay your own way. 

But it isn't entirely without value. I once held a full fellowship for study (multi year), but it still required that I spend one of those years doing the equivalent of a TA. The feeling was that it is valuable experience for any academic, even at an R1 university. And, FWIW, I actually made more progress after the fellowship ended and I was a "lowly" TA. 

But not everyone winds up at an R1 and elsewhere, teaching is more highly valued. And a variety of skills on a CV is, IMO, an advantage, though others may disagree. 

Additionally, having something to do other than your research is a useful way to let your mind rest and integrate ideas, which is a key part of learning. And a variety of experiences can be valuable in keeping yourself flexible for the future. And "more" research doesn't necessarily equate to "better" research. 

Most doctoral students get along quite well having some duties, such as a TA position, in addition to their research. 

But whether it is worth the opportunity cost or not is a completely personal thing. Maybe yes, maybe no. Think about your own needs and resources.