I just made a carrot cake yesterday and I made the cream cheese frosting without butter. I only had a little butter left and wanted to use it for cooking other stuff and my thought were does this really need butter. And I thought probably not since cream cheese has cream and fat and probably everything else butter has alone except the buttery flavor so heres what I did.
I let the cream cheese sit out for about 20 minutes then when my cake was cooled I just added the confectionery sugar and it tasted great to me.
It tasted just fine and I really didn't notice a difference.