That could actually be a pretty good point. They won't be with her, but are both with Mab instead.
Just a thought, it would not really have helped Lara to have a favour left at demonreach. When Harry had imprisoned Thomas, what would she do with it? Ask him to take him out and let him die on the beach? She could have avoided Harry using Thomas as leverage after the Fomor fight, but the main advantage of the leverage would be during the Fomor fight anyway.