I personally like the way the targeting system is now. Partially because it's what I'm accustomed to, and partially because I believe it's all that's really necessary. There's been features in every decent MMO I can think of, that lets you do your little "dual targeting", in a traditional "single targeting" interface; they're called focus frames. Having the option to set a focus frame, and make focus macros (like RIFT does), is all you would ever need. It's actually better than having a "dual targeting" system; I'll elaborate.
Example #1: In this scenario I will prove how having a focus frame is just as good as having a "dual targeting" system. So, like you said, let's say you're the healer of a group. You want to be able to attack, and keep the tank's frame on your screen, and be able to heal him as soon as he drops health, but maybe you don't want to rely on target of target to do that, because if he does lose aggro some time or another, that method @#%^s you. Well, you can simply type /focus on him, and his frame will be on your screen at all times. Then, if you have macros on your action bars such as "/cast [target=focus] Heal", you will not have to switch targets or anything, simply hit that macro, and you're healing him. Just the same as your "dual targeting" system.
Example #2: In this scenario, I will prove how having a focus frame is better, than a "dual targeting" system. Now in this scenario, you're in a battleground doing some PvP. Instead of having one friendly and one enemy target, with a focus frame...you can have an enemy targeted, and another enemy focused. In essence, two enemy targets. So you are then given the ability to make use of /focus macros for CC abilities, or interrupts, while not having to actually switch to the CC or interrupt target, and staying on your dps target of choice. This is only done through use of a focus frame.
TLDR: There's no reason for a "dual targeting" system. Sure you may have tried it, and you may have liked it because of how easy or convenient it was for you. Using /focus isn't that hard though, and anything you want from a "dual targeting" system can be achieved through proper use of the /focus system, so you really have no room to complain here, or even be annoyed, honestly. Also, like stated earlier, proper use of a focus frames allows you to do things that your "dual targeting" system does not. So I am very thankful the developers took the time to create a proper focus frame and macro interface, rather than a "dual targeting" system.
Edited, Feb 1st 2011 12:44pm by Fahros