He didn't say she's being kept alive for a stronger summon, I think all he meant was that he needed Orochimaru's chakra from her curse seal to make himself and IWR stronger, and to keep her alive because IWR requires human sacrifices.
However, I have a feeling that his intentions with her are unrelated to IWR, or at least how he's using it now. It could be that once he drains Orochimaru's chakra from her curse seal down to say 1%, it regenerates back up slowly over time, and he feeds over again iteratively getting stronger and more like Orochimaru each time. That's a major Narutoverse hax, but it wouldn't be the first. It also may have something to do with "that jutsu," which we have not seen yet, and have no idea what it does or the requirements.
If you're correct, that she is needed for a stronger summons via IWR, then it's not going to be someone sealed like Orochimaru, and it's going to be related to her having Orochimaru's chakra in her, making her a greater sacrifice. Hell, maybe it makes that IWR resurrection not suck, lol. Maybe he can make a contract with himself in case he gets killed. Who knows, interesting question though.
Peace
