John, depending on how the lintel sits leaving the joint open is not such a good thing, it can actually make a problem worse.
If the joint is caulked, the water runs down the wall, hits the caulk and falls/rolls off
If the joint is mortared, that mortar tends to 'suck in' the water, which then in turn sits there and contributes to rusting of the lintel.
As my ancient bricky explained to me (and I have seen often) the mortar between the lintel and brick tends to be less solid than between bricks. Movement, thinness of joint, temp difference between mortar and steel all contribute to that mortar being poor and absorbing water.
I know that isn't the most scientific answer but it works on the street.