why is it important that ions being transported across a cell membrane be shielded from the interior of the lipid bilayer

The answer is The ions are protected inside the lipid bilayer so that it is not pushed back to a more polar environment.

The lipid bilayer is mainly composed of lipids (phospholipids and cholesterol), and is also composed of proteins such as transporters.

The ions (which are polar) can not cross directly through the lipids because they will repel them due to their apolarity. On the other hand they can easily pass through protein channels that will protect them during their passage (the proteins are more polar than lipids).