If you like more possible rust under there then go for it. That is a marketing gimmick. "Let's hook a garden hose up to the deck and HOPE we sell more mowers". Total waste of time plus it makes a huge mess.
Negative on deck wash-out systems! They only work half-a$$ed at best and must be used every time you mow or they get plugged up up with dried grass plus...as said earlier, they make a huge mess and you'll get water in your deck spindles eventually and the bearings will be going out soon after.
Yea, if used properly. They are not perfect, but will do a decent job. You must use them after each use so that the grass does not dry to the deck. After a good 5 minute flush, you must also running the unit for another 5 minutes to dry the underside of the deck.