You will need it at times all summer. An inground pool is different than an above ground pool. An above ground pool is surrounded by nice warm air in the summer on 5 sides and is only in contact with the ground on the bottom. An inground pool is in contact with cool soil on 5 sides and air on the surface only. Thus, even in the summer you will contantly loose heat to the ground, and if you don't cover it the cool night air will drain heat away as well.

There are many factors that go into the heat balance of the pool. Wind will cause a lot of evaporation which drains much heat, the cover helps prevent that. If you really don't want to use a cover there is a product called a fish or something like that which is some type of surfactant that claims to coat the water surface when the pool is not in use to minimize evaporative losses. I have no experience with it, but have heard it is better than nothing.