Yes, because the old coolant, minerals & contaminants adhere to the surfaces inside the cooling system and water alone will not break these loose.
It would be like you working in a coal mine all day, then taking a shower with only water: You'll still be "black" (from contaminates) and you'll stink.
flushing the system is an all-day affair. Why not do it right (using Restore & Restore Plus) and not have to worry about it for 300,000 miles?
I highly recommend pulling the driver's side block plug during the drain & flush procedure to clear-out 1 1/2 gallons of liquid that are trapped in the block. There is also one on the passenger side, but it is behind the starter and would require removing & replacing the starter during each phase of the flush process.
My last recommendation would be the use of a Coolant Vacuum Refill Tool. The way the 6.4 and it's cooling system are built, it is hard to get all of the air purged out of the system. These pockets of air can cause "hot spots" and possible damage. The vacuum tool sucks all of the air out of the cooling system (it actually collapses the hoses) creating a negative pressure. The negative pressure is what is used to draw new coolant into the cooling system, which in turn eliminates air pockets. Lots of folks do the refill without the tool but I neither Ford nor I would recommend it.