Personally, I think that a flush kit and a strainer are essential pieces of equipment on any boat. As Jeff said, the strainer keeps any debris floating in the water from getting into your cooling system. It can be really amazing how much crap can get sucked into a boat's cooling system. A few years ago after all the fires in San Diego, the sea strainer was probably the only thing allowing many boats to get out on Lake San Vicente. There was so much ash and particulate matter floating on the water that even the strainer was getting clogged. I definitely wouldn't have wanted all that flushing through the boat. Even if you don't have extreme conditions like that it will catch any grass or dirt sucked into the engine.
If you go in salt water, you need to have a flush kit allowing you to hook your hose directly to the boat without having to play with a "Fake-a-lake" or similar device. This allows you to flush the boat after each time in salt water. Even if you don't go in salt water though, I'd recommend it because it allows you to easily run water to the boat when you are working on it in your driveway. Some of the newer flush kits will even allow for routing of the flush line to the transom so you don't need to drag the hose into the boat and engine compartment.
Although not necessary, you should think very hard about putting closed cooling on the boat if you plan on going in salt water. The closed cooling system will add a heat exchanger to your cooling system so that you only run coolant/fresh water through your engine block. The exhaust water from the lake or bay will only run through the heat exchanger rather than causing excessive rust and wear on your engine. If you don't go in salt water, don't spend the money, fresh water cooling will work perfectly. If you go in salt water, pony up the extra cash.