I like that "on-demand" system but see two potential downfalls. First, if the TWH has a built in recirculating pump, you would have to gut the old one in order to install the new pump. This would certainly void the TWH warranty.
Another problem is the fact that the switch, either the push button, motion detection or remote control, has to be activated and then wait an amount of time before the pipes get hot enough to use. The concept is great provided you activate the pump, walk away, then utilize the hot water.
As long as your hot water demand is at a scheduled time, I prefer a fool proof timer.