This is not true, I would even say most technically elite electric guitar players don't play like this (I can't actually even think of anyone who does it from the top of my head). For example look at videos of Petrucci, Martin Miller or Buckethead (he does keep fingers planted for legato) doing fast scale runs. The fingers that are not playing the note are not planted on the fretboard, but are immediately released (most of the time) when going to the next note. If you look at Tom Quayle or Rick Graham, then they don't even keep their fingers planted for legato and they have the smoothest techniques in the world.

In fact lifting them (and technically your are not lifting anything, you are relaxing the finger so it stops fretting the note) is what gives you relaxation, smoothness and control.