Sometimes it's the best way to do it, other times it's the ONLY way to do it!
I had a 2008 DTS for example. Took it to the dealership for a coolant change. I waited for the car. They came back to me an hour later and told me they would have to disassemble half the front end to do a drain/refill, since there was absolutely no way they could get to the lower radiator hose!
Against my better judgement, I gave them permission to use a flush machine on it. They did it right than goodness, and upon returning home my coolant concentration checked out at 50%.
Flush machines increase the possibility of introducing silicates into a Dexcool system, which is NOT GOOD.
On a 1999 Northstar STS, it was a simple process of dropping the lower rad hose and using an auto vacuum cleaner to blow out the system.
Refill with 50/50 Dexcool and good to go.
People that swear by flushes need to have the proper tool to ensure a 50/50 mixture, or whatever the manufacturer calls for. Coolant can do strange things when the mixture is incorrect.
I use a refractometer to make sure the concentration is spot on.
Ultimately you are trying to replenish the additives, so YES, drain and fill is the BEST way.