After doing it a few times, I feel like it is more effort than it's worth.
I'm making these numbers up but in theory:
Mining the rock away from underneath the entire vein involves a lot of useless whacking away at earth instead of swinging at the ore. Takes 60 minutes to collect the 100 ore that the vein has.

Instead, you could spend 30 minutes to swing at the vein exclusively and you'd get 60 pieces of ore because you didn't waste a ton of time carving useless earth away around the vein.

The first method is best if you want to make sure you get the full mineral vein without having to waste too much time searching for the pieces left unmined and time is not a concern. The direct mining method is best if you just want a quick influx of the mineral you're after and don't care that the last ~20% of the vein will be hard to track down.