I don't really Dual Client, except to play someone else's character when needed on a raid.
I don't like the idea of dual clienting. I don't like having to sit around while someone struggles to log on their second character. I don't like wiping because someone can't give one character the attention needed.
That being said, I realize how hard it can be to solo, and how unrewarding it is. I'm a wizard, and at times, with current resists and mob HP, I find it very difficult to solo anything relevant.
I would be intensely frustrated playing a cleric solo from 1 to 60. The likelihood of groups at many levels can be sort of rare.
Now I was fortunate, and via events (See Anniversary) I managed to shoot through my lower 20s. Then at around level 30 or so, I managed to get a spectacular group in First Ruins and make about ten levels in a single night. Then, again, because I had a level 60 friend who could solo very well, I did pretty well in my 40s as well.
Post 50, however, I leveled pretty quickly up until 54, when I started to completely try and group under my own power, and got all my XP from raids.
I completely see the merit, and, from a logical standpoint, agree with ending Dual Clienting. At the same time, from a player standpoint, I see a necessity for it.
Lack of grouping and the sort is just one reason we see such a lack of certain classes, like druids, who don't shine very well in raid situations.
If a lot of this doesn't make sense, it's because I'm very tired, and forgot what I was trying to say.