I would say it depends on how heavily you want to plant your tank. My 64L (see sig) is very heavily planted and I have been dosing EasyCarbo right from the start with good results. A couple of months ago the plants started to struggle, turning yellow and weak. I realised that I was simply not adding enough carbon for the sheer amount of plant biomass in the tank, so I doubled the EasyCarbo dosing and the plants had picked up and were putting out healthy new growth within a week.
This is a picture I took of some hygrophila polysperma, showing the weak growth further down the stems when there wasn't enough carbon, and the vibrant healthy growth at the tips when EasyCarbo was doubled. That's the effect it can have.
By the way the carbon in your filter does absolutely nothing for your plants - it's just for removing toxins or medications from the water.
i will say, it's probably cheaper than running a full-blown CO2 system, i havent finished planting my tank yet due to the fact some of my plants turned out to be non-aquatic, but i'll be using easycarbo when i'm done tbh
No the carbon media does not benefit plants.
I've never heard of activated carbon absorbing liquid carbon but they can certainly absorb some other nutrients so you may as well take it out. Keep it for when you need to soak up medicines or similar. Plants are very good at absorbing heavy metals and toxins so they will do a similar job to your activated carbon in any case.