Why Isabel is a Risky Choice

Spencer switching with Isabel would be a risky move because there's no guarantee that she'll help Jade and protect her with her idol. She could reject her and keep the idol for herself, even voting with Ivy, Zaid, Benji and Hannah to get her out. Isabel could also lie to Jade and claim she had already used it to save herself or someone else, continuing to keep the idol for herself. Jade will still be suspicious, but unless she finds her idol or a way to confirm that Isabel is lying, then she'll have to take her word for it.

Now Benji is suspicious of Jade and when he sees her working with Isabel, this feeling will grow towards her as well. He could look through Isabel's stuff, find the idol and steal it for himself. This will leave Isabel and Jade both vulnerable and in an even worse position because one of their enemies has a powerful item.

It would be for the best if Spencer chose someone who could weaken and distract the red team, such as Anastasia, rather than sending Isabel, who might not even save Jade.