Keeping the Contents Dry
Firstly a wet rucksack is a heavy rucksack. Let alone the wet contents adding to this total. Waterproof covers are small and light, and need to be able to completely surround your bag and any attachments. It can also be used to cover other things at camp or for kneeling on. If you look after them they can last some time. However, they are not guaranteed to be 100% protective. Wind and rain has a habit of finding every gap and these are designed in assisting to keep a rucksack dry but cannot be relied upon. In addition to which they have a habit of disappearing in a high wind, so make sure you are able to lash it to somewhere on your bag to stop this preventing.
Rucksack liners are the next line of defence and you can use these with or without the external cover. The older non roll-top type, are nowhere near as reliable as the new roll-top style. Depending on your personal preference you may wish to have one bag holding everything in your rucksack, or several medium sized ones for each group of items.
However this latter approach tends to leave a lot of wasted airspace in the sack. Some people use bin bags or plastic bags, but due to the materials used they have a tendency to rip, and are environmentally unsound when they get blown around the countryside.
Ultimately I decide what items can get wet with impunity such as mugs, plates, cooking gear and put them in a normal silicon stuff sack. For the rest, I've found a combination of lightweight compression sacks (for the sleeping bag), inside a waterproof small roll top bag acts well. This is then topped off by a waterproof rucksack cover for a 'belts and braces' approach.
However if the weather looked torrential, I would certainly consider a rucksack liner in some form as an additional option. However all these items add weight. Like everything lightweight, it's a fine balancing act.