To me, the key advantages in adapting characters from Clark's traditional supporting cast and incorporating them here is that it sidesteps a major weakness in the Supergirl mythos that has existed since Loeb reintroduced her - she doesn't have
a supporting cast. They never bothered to properly create one for Kara, compared to Peter David's run with Linda Danvers where he introduced a lot of interesting characters right from the start. I mean, honestly, who did she have in Supergirl vol. 5? Captain Boomerang when written by Kelly, and Lana Lang (also a Superman character) when written by Gates. That's it, aside from other characters shared with Superman like Lois, Jimmy and of course Alura, Thara and other Kryptonians during New krypton. And now three years into Supergirl vol. 6, they still haven't given New52 Kara a supporting cast either, her book has a reucrring character count of precisely 1: Siobhan Smythe, the Silver Banshee, not seen since May's issue. I'm not counting Michael because he's appeared precisely twice and the end of Bedard's run had been announced before Michael even showed up. Every new writer says they're planning
to give her a supporting cast, but they never actually do it, Perkins and Johnson have me hopeful for their upcoming run because they've actually named
the multiple new characters in interviews already which is more than Nelson or Bedard did with their vague waffling before they took on the book.