My horses are rugged in one way or another all year round. During winter they have their rugs on 24/7. Having said that I take their rugs off while im there to brush them and check on everything. You also have to use your judgement and if you know its going to be a warm night/day the next day put something lighter on.
In summer my guys have fly sheets on all day and then light waterproof rugs at night. You can almost set the clock to the rain during summer and it rains most evenings therefore the waterproof ones at night. If I know there is going to be no rain though I will leave their fly sheets on at night to help protect them from the plethera of australian bugs lol
It is definitely possible to leave your horse rugged in one way or another all year you just have to think hard about the weather and exactly what to put on them the night before