Not harm exactly, but it doesn't aid combustion in any way. Ask yourself whether race engines feed breathers back into the airbox...
The bigger and freer-flowing breather the better, or breather
S preferably, so that they are not 'fighting' each other, particularly in our sort of engines (lumpy great V-twins)... there's a lot of air getting pushed around under those pistons...
And naturally the 'purer' the ingested air the better, you don't want gobs of snot being thrown at your butterfly...
Engines never used to have closed breather systems, they always used to just breathe to atmosphere, it was only introduced to keep the tree-huggers quiet.