Water, dirt and oil ingress will shorten the life of the timing belt. Imagine driving down a dusty road, then u go thru puddles of mud, I dont think the timing belt would like that, esp if some large piece of stone chip gets lodged between a timing belt tooth and gear. Not to mention the timing belt might slip off the top cam gear. Other than that, I think its fine to run without a timing belt cover, why dont u go ahead and try it? :lol:
Can you do it, YES. Is it recommended, NO. It is there for a reason. If you are planning on doing this so you can show off your cam gears and you go ahead with it jsut make sure you only take off the top part of the cover that covers the cam gears. This will help things form getting to it a little bit from lower in the engine bay.