Agreed 100% on the need to keep your pack contents dry. A nice and light 5 kg pack can become a rock when carrying an extra 10 kg of absorbed water, not to mention the problems of wet gear once you get to the albergue. A pack cover or poncho designed for a pack is needed.
You probably can go without rain protection, but I think you need to be mindful about the need for wind protection. Hypothermia is just as common (and possibly more common) in the summer as in the winter. If you're wet and get a cool breeze/wind, you can get chilled very quickly. Even a brief dip in your exertion level (bathroom break, quick snack, stopping to look at a map, etc.) can suddenly expose you the chills. Alcohol, dehydration, and simple exhaustion can all accelerate the problem. Tech fabrics can also contribute because they can dry so fast it leads to flash cooling.
You won't melt from the water, but you can die from summer hypothermia. And while this (arguably) sounds more dramatic than it deserves, it's a risk to manage with knowledge. Even taking away the topic of death, a bad case can set you back a couple days of recovery. Even a light case can impair your judgment enough that you're not as careful crossing a road, navigating a steep descent on scree, etc.
The solution is simple . . . you need to buffer your core for rapid heat loss. For me, that means two elements, wind protection and insulation. (Fair warning, search these topics elsewhere on the forum, and you'll find religious wars fought over them. My thoughts on the matter are valid, but others approaches can be similarly valid.)
Wind-protection --- Gore-Tex, eVent, and similar waterproof breathable fabrics are windproof but can become bloody hot in the summer with any level of exertion. They also lose their breathability under high humidity. At 8 oz on the low end and 16 oz on the high-end, a simple jacket is a lot of extra weight to carry if not in use. A poncho in the wind defies my personal definition of practicality, but others swear by them. My personal preference is a very lightweight wind shirt that has a durable water repellent treatment. Under a light rain, they are more than sufficient to keep the rain out. They are cool in the summer heat, highly breathable (unlike the waterproof breathable membranes), and dry quickly. Most importantly, they help prevent rapid heat loss from wind.
Insulation --- I'm not talking fleece here, which will sweat out under any exertion level at all once the ambient temperature is high. I'm talking about your basic tee shirt, which (in my opinion) should be a lightweight merino wool or a merino/tech fabric blend. The reason is that wool retains its insulation factor even when wet. Lightweight merino wool is quite comfortable in the heat (though I'll readily acknowledge that the tech fabrics can be even more comfortable). Quality merino is soft and not itchy (some people do react to it, but most don't). The important thing here is that a tech fabric can dry so quickly under a breeze that you get a "flash cooling" effect, which is great unless you get too much of it. But, wool buffers that heat loss and reduces the "flash" effect.
Couple these with a true backpacking umbrella, and you can handle a vast range of weather conditions quite comfortably and safely.
