I have a keggle that I cut the entire top off of except for one of the handles. I use my old 8 gallon canning boiler lid on it. I wish I had cut out a 12 inch circle so I could use a smaller lid, and so I could could have two handles on it. Also, I have heard that leaving the curved edges of the top in place helps to direct boilovers toward the center, but I don't believe this is a real benefit. On the pro side, it is easier to see into, and might be easier to clean with the entire top open.
The first two reasons are why I will cut out a 12 inch circle in my next one.