About 10 years ago she was, pre-flashpoint. Unless something's changed in the last year in a comic I haven't read she hasn't been superewoman since then. The last superwoman was Lana Lang, whose series ended a while ago. Haven't seen Lana around in a while either.
I don't think that's the reason she hasn't been used. It's a combination of editorial mandate - at least from the era when Didio was in charge, and a feeling that she wouldn't sell even though her series sold as well as anything of Kara's many series have over the last 15 years. The writers probably also have a general lack of interest in writing her. If there was a fan campaign like there was for Stephanie Brown and Cassandra Cain she might have a chance.
If we here can come up with a bunch of ideas for how she could fit into the Superman mythos, from her merging with a Kryptonian deity to her working primarily with Kara to the original pre-flashpoint Linda just showing up with Ariella in tow, then the professionals can certainly find ways.