I usually always ride with gloves! They give me a little extra grip and put up a barrier between my skin and the reins. You can get some nasty blisters and tear your hands up pretty badly is you are working hard without them. However, if you are gust going for a light hack or trail ride and it's super warm out, wearing gloves isn't such a big deal.
I always do because most of my horses are pretty strong and if I don't then I will get blisters on my hands. If you are not competing and you don't have to. If you have a strong horse so would always wear them. I like to regardless cause I think it looks better and the gloves are grippy which helps avoid when your reins slip.
I always use gloves. During the summer my hands sweat a lot, but it depends of the fabric they are made of.
There are lots of brands which are selling summer gloves made with thinner and cooler materials. :)