One advantage to keeping it in tIRA is that while you pay normal income taxes on the amount you roll over, you can "roll" the capital gains/interest and use that as part of roth pipeline. If you rolled it over now, you can't touch that without the penalty for early withdraw.
I do use a roth IRA and count on the $6k contribution as part of my roth ladder conversion priming. After 10-15 years, I'd have a $60-90k prime... not enough for a full prime for me. If I rolled into it now, I can get it to $100+ plus, but still won't be enough for me. But instead, if I keep it as tIRA for now, and until a few years over, used that to prime my roth IRA ladder, I will have enough. Once that is done, the 401k will take over.
This might be a tax inefficient way but I feel better having this setup than being "locked" out of my ladder for a full 5 years... And my 401k doesn't allow me to make withdraws while working to start ladder either and I don't want to be without ladder when I start retirement. I may not need it, but I want it primed and ready in case I do need it.
for me, it's the "end plan" of having a functioning ladder on retirement date that I keep rolled 401k as tIRA instead of rolling into current 401k or converting it to roth now. I plan to convert it later once the value of it grows to make it be a better prime.
Taxes while important, isn't the biggest thing for me in this regard of picking what to do with old 401ks. It's what I plan to do with them that matters more to me. And I want them as primers so I want them to grow and be able to use the entire value for the priming. So they stay tIRA until that time.