My DD has done it. Hawaii, New Orleans and two gyms in Ireland

Hawaii was a private - we were gone 10 days right before her last meet, and she had some lofty goals for that meet. So we got her a private and tweeked a couple of things.
New Orleans was a group practice that was beneficial because she was just starting to get her kip, and she didn't want to lose the little bit she had while we were away. She had a great time, and still remembers a few of the girls.
Ireland - we were gone for almost 3 weeks, and just like the kip, she was starting L6 skills, and didn't want to lose the little bit she had. And because she thought it would be cool to train with girls who sounded different than her. (Her social takeaway: "those girls say the F word an AWFUL lot!"

)
It's not for everyone, and everyone who does it has their own reasons, whether gymmie wants to do it for fun, or wants extra practice, or wants exercise that she doesn't have to drive herself. And truth be told, sometimes if a skill is close but not quite there, sometimes fresh eyes can help, but it's not like you can just go down the street at home and have someone else coach your kid.
Funny thing about my gym - never once did we ask if it was okay - we told them she was going to do it (first time was old level 3, so not a huge deal), and they thought it was cool. We consistently have a couple of girls from Puerto Rico train in our gym each summer for a little bit, and also a couple of college girls who have come in during summer.