Ah yes, dual wield does have some associated shenanigans, as both of you explained so well. There's another interesting mechanic that happens post CM 1.60, and by extension on PFP as well, and it has to do with the new behaviour of mage staves. If you dual wield a staff with any other melee weapon, and you use the staff as your main weapon, the attack will actually be... ranged! Since staves were changed to shoot projectiles even without its relevant weapon mastery, but still count as melee weapons, you can dual wield one with any other melee weapon. This can result in some really wild results if you run with a Malevolent Champion SW and modify Frenzied Rampage twice for double hits. Want to see something fun? Try it, it's a blast. One-handed staves don't have %LL, but if your secondary weapon does, you can then throw a barrage of projectiles and each will proc it. RpH gets pretty crazy as well, so you can get your CA's cooldown really high and it'll reset very easily.
As for DoT builds, they do seem to be more effective with LMB attacks if even multi-element CAs can only proc one effect per hit. It's probably possible to theorycraft a build that relies more on CAs as support instead of using them as a constant means to attack, in order to maximise DoTs. I remember that there was this crazy Spectral Hand build back then with Damage Lore maxed out, and the guy who ran it used to farm the Guardians with it. It looked like it was a lot of fun, there's probably still its forum post around here somewhere.