Using shields with versatile/two-handed weapons?

There are several members of my 5E party who use versatile weapons, such as the warhammer, which does 1d8 damage normally but can do 1d10 damage if wielded with both hands.

One of these members also has a shield, which gives +2 to Armour Class. Shields are carried in one hand, and as such cannot be used at the same time as two-handed weapons.

However, when in combat, is there any reason why my player cannot at the start of her turn put away/drop her shield, attack with her versatile weapon two-handed, and then take her shield back out/pick it up? To me, this feels like a loophole with versatile weapons and shields that allows her to always benefit from both the AC bonus of shields and the damage bonus of using weapons two-handed.