I don't usually use one in aiki-ken, but we rarely do anything that has us making more than incidental/accidental contact with our weapons. If I was in a style that did a lot of contact I'd definitely use one. On the occasions when we do jodo kata at my iaido class we always use a tsuba on the bokken. Back when I was doing medieval armed/armored combat, good hand guards on the weapons (or good hand armor, which is much more expensive) were always extremely important.
I don't have a problem with them. I don't use one because that's the convention in my dojo....and because it fits in the bag better without it