For my 9mm build I am contemplating using a spare flatop 5.56 upper receiver.  I was going to sell it off, but I figured if I could use then... well you know.

Anyway, is the 9mm brass deflector needed?  Anyone have any experience shooting one without the deflector?

I'm not sure if it matters but I am planning to use a 3 lug barrel on it for a suppressor.  Someone mentioned to me the deflector mainly prevents debris from hitting your face moreso than being a brass deflector.

Thoughts?

As far as brass deflection goes, you will need one if you're a lefty.  As far as gas and debris goes, yes, they help a lot.  The 9mm is a straight blowback design and with a can mounted, you DO get blowback.

If your a righty....NO...I have had no problems with blow back and I shoot a lot....but not full auto...

I have shot a lot of full-auto out of mine and don't have any issues or complaints about debris or blowback.  No deflector or suppressor.  Most of the people who have shot it are righties and the one leftie who has shot it didn't say anything either.  He is also a MG-guy.

on my 9mm carbine the brass deflector still looks new the brass doesnt even hit it.

As it has been said above, the deflector is not necessary.   I think it is a great visual aid  to differentiate between  the .223 top end and the 9mm.  I think it would be bad to mix them up if you swap uppers on a shared receiver!