I play D, and I strongly disagree that turrets and/or walls are useless. I get a lot of comments about using forcewalls around the node, and turrets, and I have to say, they are CRITICAL for good D if you don't have several players hanging back to lock the node down.
Turrets cause a little damage, which is useful, but not why I primarily use them, unless I've set up an arrangement that can actually keep invaders busy while the turrets chew them up like you can on some maps. My primary use for turrets is early warning; even if they get blown up before they cause one point of damage, now I know there is an invader near and where they are, and I can be prepared to meet them or flank them. That is *invaluable* on *many* maps.
Likewise, this game is all about timing. Force walls around the node in a box (not the two walls inside the node arrangement, but in a box around the whole node) do one of two things, or sometimes both: first, they slow down an attacker and may entice them to blow the walls. Again, now I know where you are and where you are coming from, or if I already knew that, you aren't shooting at ME for a few seconds; I can't count the number of times that few seconds is the difference between you grabbing an art and getting out before I can react, and me smoking your ass on my node platform.
The second thing that arrangement can do, and often does against players in a hurry to grab, is it makes them burn either one (if they blow one side of the wall and exit the opposite side) or both boost-jumps they have available (one to get in and one to get out). That is *incredibly* useful. I can't stress it enough. Even if you're a ranger, burning those jumps out means I can stay with you and hammer you down with my weapons before you can recharge a boost for yourself, in fact most of the time I'll have a movement ADVANTAGE as a tech with boost jumps vs a ranger without. Again, I can't count the number of times this tactic has made the difference between a player boot-scooting with my arty past my shotgun range in seconds and me sticking to them like glue to hear the glorious sound of an artifact returned.
Always remember, seconds make differences, especially when it comes to grabbing artifacts. If I can slow you down a little going in, keep track of where you are, and keep you from blasting out of town at top speed, I'm keeping my artifacts safe, if you manage to grab it at all. It's not foolproof but it works WAY better on most maps than just sitting around as a gunner.
Of course if you have multiple defenders and you're organized at all (watching separate entrances and such) the tactics you use will be different. But as a regular sole defender I have to say both turrets and walls are *incredibly* useful, even if their use isn't readily apparent to others or isn't directly related to killing the invader immediately. It's about setting up a situation where I can attack you with advantage and take you down, as opposed to getting caught with my pants down.