The problem with an air brick is, if you leave it unblocked there will be a howling gale of cold air forming a path between it and the extractor fan when the fan is running.
You will also have the problem that at times your warm air will be disappearing out through the air brick and at other times cold air will be sucked in, depending on which way the wind is blowing and how many holes you have in your home.
From 2016 all new homes will be built to Passive house standard that means as air tight as it is possible to make them. No airbricks or trickle vents etc.... NO HOLES!
Keep in mind that every time someone fits an extractor fan it has to find air from somewhere to expel and that means cold air is pulled in somewhere to replace your warm wet air that has been blown out.
This costs you, every time you have to warm this new cold air.
A better bet, is to fit an extractor fan fitted with a heat exchanger, this pulls in air that has already been warmed by the outgoing warm air, no drafts and maximum savings.