Mixing saltwater: it's easy and convenient. It's often even more economical than buying it, especially when you have a large aquarium.
I hate to say it, but it's been over two years since I bought premixed saltwater. It was a chore to drive to my local store and then come home with mixed salt, only to then begin the water change. The chore of going to get the water was my old excuse for not doing frequent enough water changes. (I have now developed more complex excuses... Did you hear about the time my RODI unit was sabotaged by a band of of rouge agents bent on world domination through the proliferation of cyanobacteria and destruction of coral reefs?!) Well the moral of the story (the purchasing of water not the rouge agent thing) was that its a mission to go get water, especially if buying large quantities of saltwater.
Besides the chore factor, the costs can add up quickly when you're paying someone else to mix water for you. While I can concede that it's less equipment and cleanup to just bring a couple of containers for fill up, I would not want to have to buy 30 gallons per week to change water on a 300 gallon system.
There is also a final consideration: emergencies. It's easier to always have a little saltwater on hand when you can mix it yourself. This is good in case something unexpected happens- and "it" will happen. Having water on hand might be the factor that saves your reef from failure.
