It's terribly late but I want to get this out before I sleep.
Theory: The current way EXP is calculated (Additive separate bonuses and 20% Party Play bonus compared to Multiplicative combined bonuses and 50% Party Play bonus) is a somewhat sophisticated solution to the "5 leech, 1 attacks" party line up that plagued LHC and HoH all this time.
I'm thinking that because the Party Play bonus is 20%, the maximum bonus (6 person party) would be 100% which is just below the full exp penalty for having 5 party members. This means that if a party followed the "5 leech, 1 attacks" line up, the attacker would be hurting way more than if they were to just solo, even if they were getting HS. The only benefit is that the 5 leechers still get EXP for doing nothing but this solves the older problem where the attacker wouldn't have cared as the party play bonus was still way faster than soloing (without a party).
The other part is that the current EXP modifiers (HS, 2x Events, EXP Buffs, Item EXP buffs, etc) are additive now so as not to disrupt the above solution. If Party Play EXP was multiplied by our current EXP modifiers, it would then suddenly create the incentive for a "5 leech, 1 attacks" party as it would be faster for the attacker to leech a party. Currently, EXP modifiers are solely modifying the base EXP of the mob with Party Play EXP as a separate bonus. By doing it this way, it will still impede solo attackers that leech their party members. I'm not certain as I haven't done the math quite yet... but this would also mean that having a 6 person party isn't imperative and that having fewer party members might actually be better assuming they are active. The system benefits stronger players that only require a few people to train on a map and it benefits weaker players that can't kill as fast but can remedy that by having a larger party.
This is quite brilliant (if it was true) as the way to counteract these penalties is if everyone in the party is actively attacking which would then make it faster than solo play, thus promoting true party play.
If someone can be so kind as to help me do the math and calculations to support this theory, I'd greatly appreciate it. Tell me what you think in the mean time. I could be wrong but it was a nice thought :v. Anyway, going to bed...