Testing and developing for the new Advanced Trade API is a bit anxiety inducing since you have to use real money and create real tax implications with code that may have mistakes. Getting a sandbox API would help with this.
I think I speak for many when I say portfolios will be dearly missed from the Pro exchange for multiple reasons.
But for myself, portfolios could also help with development in lack of a sandbox. If I could add another portfolio, I would only have to risk say, $50 on untested code, instead of literally all my funds when it gets transferred from Pro next year.
Also, I’m building software that largely takes full control of my account. I want to run that software on a different portfolio so I don’t have to risk all my money and effectively lock up my account. I also want to continue developing that software while I’m using it, but I can’t do both on one account. I need either a sandbox, or an extra portfolio or development will freeze when I start using my software.
It would also be nice to have multiple portfolios so that once I think my software is stable, I can test it on a smaller account with real money. The sandbox API for Pro never reflected the real world very well, and if no one else was using it, it was effectively useless as there was no one to trade against.