A breather gives you a break. I often take them while playing hockey.
I'm guessing that you're talking about bypassing the PCV valve re-route to the intake?
The fumes that the PCV system lets out of the crankcase get routed back into the air intake so that the engine burns them completely, helping emission control. But that "air" that's getting sent back to your intake has oil mist in it. Some people think that it's totally evil to let this back into your engine. There's also a case to be made that fresh, clean air is better to have going into your engine than some air with some fumes in it. I don't think any of us will notice that perfomance degredation...
Positives: no oil vapor from valve cover going back into the intake
Negatives: breaks emission control laws, might void warranties
I'm not going outside to look right now, but I don't believe that there's a filter at either the PCV valve end or at the other end of that hose as it heads back to the intake. I'll look tomorrow if no one else jumps in on it...