I wouldn't think a person would even need an underquilt in the summer
It depends on what the rest of your setup looks like. If you're a guy who likes to use a sleeping bag all year and just add a UQ for colder weather, no you won't need more than that in the summer. I personally like to have nothing on top in warm weather, but have found the need for a little something underneath when overnight temps have gotten below 65-65* or so.
"Physics is the only true science. All else is stamp collecting." - J. J. Thompson