I was told that it allows bacteria in. Right now I have 2 very porous eggs in the bator and they have moving babies inside. They are on day 11 today. I won them on E-bay - that's the only reason I set them. I personally wouldn't set any porous eggs from my own flock.
Depending on how porous the eggs are, it just means that it may increase the chance of infection. However, the biggest thing in my opinion is that these eggs will dry out faster while incubating so you have to watch their air cells closer so they don't lose too much water. Conversely, if you set thick shelled eggs with porous ones, watch the thick shelled ones for not losing enough moisture.
Yikes, I hope they aren't too porous. I have a doz. eggs whose shells look fantastic, and then six from another breeder that look...really speckly. I was pretty surprised and had only candled one for the heck of it, to see what I could see. They aren't as bad as some pics I was able to find on these forums, so maybe they will be OK.
What causes porousness in eggs? Is it diet or genetic or something else?
I believe for the most part it's diet and other factors, like molting, which causes stress. From time to time, a certain girl will have slightly porous shells then they'll go back to normal. I know this because I try to candle eggs before I ship them out for hatching. Slight porosity isn't a big issue. Swiss cheese may be a problem. It lets out humidity too fast and bacteria can enter easily. I still would set them because they can hatch just fine, depending on just how porous they really are.