The breather cap is gone from the vent pipe outside the house. Do I really need to replace it?

Yes, you should replace this cap with one that is designed for that purpose. If your vent is left uncovered it can easily become plugged up with debris and not function properly. This is also true of the clean-out caps in the yard. You should also check the vent pipe in winter to make sure that snow has not covered the top, which makes the vent useless.