I have never considered running without a valve but I suspect you'll get fuel fumes wafting around in your garage without a valve.
The Marlin valve is to let air in as the fuel is used up but doesn't let the air out when the fuel expands, which it does alot!
Previous discussion from a few years back here....
http://www.madabout-kitcars.com/foru...referrerid=181
I fitted the merlinmotorsport valve and have had no problems since.
...peter