Since I learned gybing and boards are getting smaller I almost completely forgot and lost tacking ability.
How essential is this skill or we could live without?
lots of people seem to do fine without being able to tack... depends on what kinda sailing you do.
i think it's pretty important if you mainly sail waves or freestyle, it means less time spent sailing upwind.
Nah I wavesail and have never been able to tack. Have no desire to learn, all the falling in will mean missed waves 
You don't end up that far downwind if you just stomp on the tail and screw it around on the spot (as long as it is windy enough to not need to bear away to plane again)
It is a good tool to have in your bag for sure!! On a shortboard, you will need the help of backwind your sail to keep you up at the beg, so trust the force....
I found tacking on short board quite difficult so every time I need to consider:
if better is to loose a few meters on quick jibe
or
risk water starting after failed tack and loose even more in our strong current ( up to 8km/h).
But I do admire those that could perform nice tack in wavy or surf conditions 
Agreed Macro.
I find losing 5m downwind on a gybe is of no consequence if I get 50m upwind on a reach and that is enough to be back upwind of the right spot for the next wave.
As much as redthumbs disagree (?)
You are talking about essential and I don't think it is essential.
I still wish I could tack well, but can't be bothered learning as I've never felt I have to
a fast shortboard tack is a thing of beauty, unfortunately I can still only do them on one tack. You can live without the skill but I must admit at places like Hookipa you feel like a bit of a dill gybing on the inside when everyone else is tacking close to the rocks
G
^^^ +1 not essential to learn but after you do the first one under the shadow of a big rinse coming you may think they are bloody useful
Not being funny here but have always found heli tacks easier to do on a short board in light winds, I sink the front down if I'm not planing
