My own feelings are that I don't use it, but I also don't think it's an exploit. Here's why:
1. Tab targeting just fires spells in a straight line at the target you choose. It doesn't (and won't) aim at weakspots. It doesn't correct trajectories for launch rock. It doesn't "lead" monsters that are moving. It doesn't make even slight corrections to avoid hitting terrain. It's _just_ a straight line, and that's all it will ever be (no improvements are coming, since those really would be auto-aim-ish).
2. Given the above, that means that you've traded one kind of aiming for another. Instead of aiming your cursor at the enemy, you're positioning yourself so that the line between yourself and the enemy does what you want. That's harder than it sounds, in a lot of cases, because you're also dodging enemy fire and dealing with terrain and so on. But for some folks on keyboards, or some folks who are less twitch-style gamers, moving themselves and their cursor is out of the question, and just moving themselves in relation to their enemy is much better.
3. Tab targeting is inherently slower. You have to cycle through targets, and in particular lining up really sweet piercing shots that hit many enemies at once is way harder to do this way. In other words, tab targeting does give some advantages, but it also has some disadvantages, which seems about right to me. This is what makes it just an alternate mode of play rather than an exploit. But when you get right down to it, an adept player with a keyboard and mouse setup is always going to be better at the game than an adept player playing using tab targeting. This, again, is as it should be in my view.
4. Compared to "movement directed aiming" as a keyboard-only way to play, tab targeting also has advantages and disadvantages. It's still slower to use, but it's able to hit angles that simply aren't possible with the movement-directed aiming. You can definitely position yourself to hit a weakspot on an enemy in either manner of play, but in both cases you have to put in the work to line yourself up to do so. Thinking that tab-targeting is like auto-aim in an FPS is definitely not correct, in my view.
To me, there's not a problem here. But there's been some grumbling about tab targeting existing, as well as some requests for enhancements to tab targeting that I definitely don't want to do because it makes tab targeting too powerful, so I felt it was a good idea to at least open a discussion.
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