Urea is a product created by the liver from nitrogen that would otherwise, since it is being elementally broken down, become amonia,a real toxin.
HIgh uric acid concentrations in the blood stream have certain negative qualities,tho not considered exactly toxic, check out the harvard studies on vegetariansm, but as mentioned already the ureas components can be used in reingestion by the liver to build other compounds, and is discharged in urine only because at that moment it isnt needed.