Mine isn't plugged and it runs great.

 

Reason there is a vent hole there is so various gases coming up from the crankcase, via the distributor shaft, have a place to go. I have seen unvented caps that trapped combustibles in them, literally explode, as there seems to be a rather frequent ignition source there...... Of course, if it ISN'T vented, you would think you would have less gas intrusion there. Not sure if that translates to less oil getting up there as well.... I haven't seen a distributor fail due to lack of lubrication, unless the WHOLE ENGINE followed on shortly thereafter.