Some people like to learn the hands seperately; I've never seen the point myself.
To be honest, I think there is a really strong case for practising separate hands unless you can say, without a shadow of a doubt, hands on heart, that you're left hand could play the treble clef (right hand) notes as equally well as your right hand. This, alone, would make me practise the left hand with my full attention and care. Having said this, it also depends on the skill level of the piece being played. If the left hand notes are easy to play, then I agree, this is certainly so in quite a few pieces if the notes are patterns which you have played many times before. If this is not the case, then separate hand practise is most definitely helpful and worth the extra effort.
This implies that the "difficult" stuff is in the RH and the left normally gets a free ride. There are no doubt many pieces where this is true, particularly in the intermediate level, but it is not generally true early on or in the advanced repertoire.I also don't understand why one can't practice the LH (or the RH) with "full attention and care" while still playing the other hand.
Are you saying that you have NEVER practised your left hand alone? Ever? Even in more advanced repertoire? I did plenty of it while practising Dohnanyi's Rhapsody in C major for a concert.
I should add that I am aware many people find HS practice useful. I am not against it, I just personally don't get it.