Hi Sanjai
I cant share the names of the clients here, but the project I am working in right now is an automotive major and they also use the same approach. Earlier also I have seen this approach and this is one of the approaches recommended by SAP
When you use a single BA for IGAAP postings and simply run report at Company level, it will show you report for the whole company and not just IGAAP in my opinion.. Then you will have to add one more BA for US GAAP postings.. Moreover, BAs are not flexible enough and not scalable incase your future requirements change
Br. Ajay M