Because we live in an era where we are constantly exhorted to get in touch with our inner this or that and then let it all come out in one great rush of emotion, it certainly makes sense that we should follow this dictate in relationships. We should just get everything off our chests, unload any thought or feeling that crosses our minds, hold nothing back, all in the name of openness.
The problem is that totally uncensored venting of your feelings often just does not work. Think about the number of times that you have blurted out something in the heat of the moment about your partner's weaknesses. For a moment you felt the exhilaration of rage - and you quite possibly damaged your relationship, and sometimes the damage can be permanent.
I have seen many a relationship destroyed because one or both partners simply could not forgive something that was said in the process of venting. "Once you've said something, you can't take it back."
Before you say something that could be disastrous, you must give yourself breathing room, you must bite your tongue, you must allow yourself time to deliberate. This absolutely critical to the future of your relationship. I'm not telling you to hide truths and be dishonest. But I am telling you that in order to meet the criteria of being open and honest, you need to be sure how you genuinely feel, you need to know if what you're about to say is going to be said in the most appropriate manner, and that may take more deliberation than is available in the heat of the moment.
I also want to point out that venting is not only verbal. Oftentimes, deeds do in fact speak louder than words and are as much of a communication as any words you could ever speak. Slamming a door in your partner's face, walking out at a critical time, throwing a drink in your partner's face, shutting out your partner, or failing to be there when he or she needs you can all communicate powerfully destructive messages.
What's more, you need to be doubly careful of the way you vent when you feel you have been wronged by your partner and you want that partner to know it. How you respond can cost you a lot of credibility and completely take the focus off whatever your partner may have done to hurt you.
Don't be a sucker and get baited into going off the deep end in the name of some misguided self-actualized self-expression.