You don't put drains in the block for flushing, you put them in there to DRAIN the block. You know, get all the coolant/water out of the block. So you don't drool coolant all over the place when you pull a head or something similar. Pretty sure the Prestone flushing kit doesn't get the sediment at the bottom of the water jackets out either. Since I've seen it come out my drains after a flush, draining all the water out for a proper refill.
Petcock draining does not always if ever remove all coolant from the block,,,just ask some of those unfortunates in cold climates who have drained their blocks only to find a cracked block when a small amount of residual water has frozen,,,,just as freeze plugs do not protect a block from frozen coolant caused cracks,,,as most know they are there only to fill the holes that are there to drain sand from the block post casting.
Mere draining also does not normally do a decent job or even a fair flushing of a heater core.
Additionally the Prestone unit described virtually eliminates the all to real possibility of leaving vacuum chambers in your cooling system when filling it.
Flushing and draining both are the best approach of course. How much time do you have?
