Our group tags enemies all the time, artillery or not! We typically tags enemies in order of priority... *ALWAYS* tag the turrets and heavies first, then sometimes tag missile trucks and mine trucks especially if we think we might need help, then hoppers and scout bikes if we're bored or want targeting range info on the computer (below the radar).
A useful reason to tag enemies, the others in your team can see them through walls too... so it's a very useful strategy! Especially with the nemesis tanks. We tag the strongest enemy we see first and call it out on voice chat as tagged so everyone knows what to look for and where to direct their fire.
I agree with the thought on enemy tanks, they should not become transparent allowing other tagged enemies to be visible through them. But I guess I'd like to be the first to make the argument that hackable buildings should. :)