Yup, that is correct. So if 6 slotting a toggle (which is a big investment), the question becomes how much do you value the damage over the set bonuses (4-6 in this case).
As for why people dont like to slot in auras? It depends, I suppose. 10 seconds is a long time to wait for a proc if you are railroading mobs (but if you're railroading mobs, do you care?). Also, there is no exploitation with toggles like what you can do with long click powers. You can have enough global recharge to proc out a click power for reliable DPS, you dont get that with toggles.
Finally, let's go back to those 3 slots you spend on the aura. Does 66 damage per 10 seconds per target (6.6 dps/tgt) make it worth it to spend 3 slots on? Depends on your build. With some builds being so tight, it might be too difficult to squeeze out that little bit of extra dps.