It's actually a really useful support tool. It can paint over a zone to retake it, alert your backline to flanks, weaken enemies moving through chokepoints and give you a quick defensive tool if you have a slow main weapon.
Ink Mines can even have a use if they never go off. Since it's still there, that must mean that nobody is in the area and you don't have to watch out for someone flanking you from that area.
BUT, It can be very helpful if somebody's doing a sneak attack and you main e-liter. after 2 of your mines hit the player, tap shot them. If you don't use it right, it is useless
With proper strat, it works amazingly
If you wanna practice with it, I believe splat3 has a level in story mode where you have to use the inkmine to kill all the enemies