As with all of the X-men training upgrades, it's a pretty good value over time. Imagine the smallest ally, someone like Forge, who usually thwarts for one and then blocks. With a Mission Training, now that ally can thwart three times for a total of six threat removed, and still block. That's an increase of five threat removal, for 1+card, which is better value than you can get almost anywhere else, though it does take several turns to receive the full value. For bigger allies (higher THW and/or more HP) you can see even bigger gains for the same price.
Attach to an X-MEN ally. Max 1 TRAINING upgrade per ally.
Attached ally gets +1 THW point and +2 hit points.