That's not a buff

Tibbers gaining a very quickly decaying ms buff and molten shield when Annie uses her molten shield is not a buff, it is compensation for her losing 10 armor and mr from her molten shield at all ranks.

It's compensation for her losing ~5% magic and physical damage reduction when using her molten shield at level 3.

At level 3, molten shield's armor and mr buff gives ~10% magical and physical damage reduction. With the changes her molten shield is half as effective early and she doesn't gain anything from the small tibbers buffs until level 6.

This is not a buff, it is a just a small trading of benefits or movement of power that she doesn't get access to until level 6 and only when her ult is up/out.