Let it breathe!
The argument for those in favor of leaving brick exposed is based on the way that walls are designed. A professionally designed brick home has a cavity between the brick outer leaf and the inner leaf of the home. This cavity prevents the moisture that penetrates the outer leaf from getting through to the interior of the home.
With this design, the brick gets wet during wind driven rain and dries out once the storm has passed. The water gets into the brick but does not go any further into the house. The cavity prevents the formation of mold in any construction materials used for the inner leaf of the house.
Of course, as a homeowner you are leaving the brick and mortar exposed to the elements, so they will deteriorate over time. The effects of wind driven rain and snow are most pronounced in the joints themselves; you can find the mortar recessed almost an inch from the face of the brick on older homes! However, you can also see the wear on the smooth surface of the bricks.
In addition, there is a risk of moisture remaining in the bricks, and a sudden temperature change can cause that moisture to freeze. The expansion of water during freezing can lead to small failure points in the brick and mortar that become more problematic over time.
Seal it up!
Those in favor of sealing exposed brick recognize the toll that mother nature takes on their home. They are looking to slow that wear and mitigate future costs in tuckpointing to repair the mortar joints as well as prevent any structural failures.
A fresh brick sealant properly applied also prevents any concerns with mold forming, as well as the cosmetic benefits of slowing the formation of alkali and efflorescence on the bricks themselves.
Problems arise as the sealant wears away unequally over time. This means that parts of your brick will start to breathe again while the majority of the wall remains sealed. This causes the brick to dry out slower, thus increasing the risk of mold forming down the road once your sealant starts wearing away. The wear won’t be obvious to the naked eye, so the risk is more subversive.