I use Defoamer myself. There are a few things to keep in mind :
They contain Silicone and Surffactants. Whats that mean ? You gotta pay attention to the dosage as there is a toxic limit per FDA regulations. It will add to the bitterness of the brew somewhat but not too much. Just READ the directions and use it sparingly.
The other aspect of the surffactant is that it modifies the surface tension of the wort. For foaming that's great, if your using Whirlfloc don't expect the same results you would normally see. The Whirlfloc will still catch the big goop like hops but as for coagulating the proteins... you will need to filter to get the clarity.
At the end of the day, I use it. I am careful though and I balance the clarity issue. If I want to make stuff that looks as clear as Corona then I know I have to filter. If I'm hopping the hell out of my wort and just intend for the whirlfloc to take care of the hop goop then I'm good without filtering.
Just some observations.
Oh p.s... this could be just my imagination but when the lauter FIRST starts to generate steam off the initial boil and you add the Defoamer... stay clear of the steam for the first 2-3 minutes. It's almost like tear gas or some sort of Mace. it will burn the bejesus out of your eyes and throat. Once it's rolling though your good but you'll still smell a little chemically tint in there through the full 60 minute boil. If you eat organic spinach then I'd just bag it and get get a bigger pot and try to throttle back the gas to prevent boil over.