Well, let me give you a situation from last night, since I experienced this exactly as people are talking about it.
Level 21 Cleric, level 9 Rogue. Zone into MielA; they are NOT grouped. Rogue pulls, but gets both of the front-room mobs, AND the wanderer... so 3 mobs. Rogue begins to kill the mobs. Cleric drops a heal on the Rogue. The two mobs the Rogue is NOT hitting run to the Cleric and start trying to hit him. The Cleric just mindlessly watches them flail at him while healing the Rogue. Rogue kills first one, pulls one of the others off of Cleric, kills it. Cleric has healed Rogue at least 2, maybe 3 times by now. Eventually Rogue kills off all 3 mobs. Rogue has received maybe 4-5 nearly full heals from the Cleric through the fight... all 3 mobs give Rogue 100% full experience (no penalty).
This is how things were working last night. I would think at least for that last mob, my Cleric would have had significant aggro built up... but perhaps not. Regardless, as much as I LIKE the way it worked when I tested it, my guess is that it is not supposed to be that way (again, just from hearing people mention Anti-PL code). Oh, also tested with a lvl 18 wizzie putting a damage shield and the blades of frost proc on the n00b rogue and an enchanter putting haste on him. I was under the impression that at LEAST the damage shield and the heals during combat should have caused experience penalty. Note, I didn't continue with the wizzie, enchanter and cleric all assisting the Rogue like that... I just wanted to see, after the heal thing, if ANYTHING would cause assistance penalty.
Is it possible that there's... I dunno... an allowable level range? Like, if the toons are within 20 levels of each other, no assistance penalty is given... or something? I dunno.