I've had computers with and without missing PCI clot covers and worked on many as well. I've never seen it make a differnce. I will say though that my new system I built last week has all the covers and I'll be sure to make sure it always does.
If the open pci cover is not within any airflow, I would try and find something to close it. Dust and other stuff could just float on in w/o a care and lie on components.
If the open pci cover is not within any airflow, I would try and find something to close it. Dust and other stuff could just float on in w/o a care and lie on components.
Apart from having to clean it more and a bit more heat there's no problem with reasonable amounts of dust building up, missing PCI slot covers shouldnt let that much more dust in