Really sounds like yet another unnecessary feature. If you're out and about, you're on mobil data anyway so wifi doesn't even come into the equation. If you're in a wifi area, in almost all cases the wifi provides a superb signal rendering any presumed benefit of "adding" 3/4G service to it moot. I knowe when I'm at home on my wifi network the last thing in the world I want to do is to (unnecessarily) burn through data to speed things up from the light speed I'm already getting.
Seems like a not too clever ruse on the part of the carriers to get subscribers to burn though massive amounts of data so that they can charge them for this.
No thank you!!!