With Kompany's future, let's say, uncertain, we need 5 CBs.
Given that we don't want a mammoth squad with certain players never playing and costs needing to be taken into account, some of these 5 CBs will need to be versatile enough to play other positions - some of which good enough to start in these other positions. This is something Pep wants anyway.
Mangala and Demichelis will leave so we're left with Kompany and Otamendi.
Denayer will return from loan.
Laporte will probably be signed.
That gives us 4 with Denayer able to play RB (and, I think, start there)
From what I've seen I think Stones can play DM. And that, as well as his age and potential to be part of a very long term back 4 with Denayer and Laporte, is why I would sign him.
Furthermore, Stones sat infront of a back 4 of Denayer Otamendi/Kompany Laporte and an attacking left back, would give us the option of switching between 3 and 4 at the back with Stones simply needing to drop in.
Everton promised him he could leave this summer. I would do the deal. People were slagging off Barkley this time last year after a poor season. Some silly things were being said. A few months later, before Everton downed tools under a dead man, he was on fire and BM was singing his praises. I think a similar story will occur if Stones ends up elsewhere.