I do understand that's the way the current system is. I also understand that yes, in general, a level 80 character will improve the 'power' of a group moreso then a level 75 character.
However, the current setup is overly harsh, in my opinion. If you have five level 74 characters in a group fighting white con mobs, one might say they are taking on challenging content and the xp gains are commensurate. By adding one level 80 character, that 'white con' xp suddenly becomes 'light blue con' xp. To get back to the level they were at before, this group would have to start fighting level 80 mobs (ie. white con to the highest level).
Now a case could be made this is fair from a design standpoint. In other words, it looks good on paper. But have you EVER grouped with someone higher level then you? Did you consider it an exploit to have a character 6 levels higher in a group with you, such that you would agree to have your AA xp output cut, perhaps by a drastic amount?
Everquest is a social game. Part of the fun, for some people, is meeting new people. However, Everquest is also an older game, and in many ways has become more clique-y as time has gone by. I remember Back In The Day that I could log in and get a pickup group relatively quickly. Last time I logged on (a few months ago), I remember being on my shaman casually soloing in Loping Plains, do some orc/werewolf quest. I had my lfg on. I finished the quest, tore through a lotd, played for another little bit, then went afk. When I came back, I had been online maybe 90 - 120 minutes. During prime time. As a shaman (a relatively desirable class). I hadn't had one tell asking if I wanted a group. Now yes, I know, make my own group. Join a guild. Don't suck so much.
But what it came down to was that for about 2 hours during prime time, I wasn't invited to a group as a shaman. This was my experience more often than not (as a full time casual player).
I can't help but wonder if some lower level groups bypassed me, because they were working on AA xp (these days, even by the devs admission, you need 500 AA at level 80 to be 'tuned' for the easiest zones), and didn't want a level 80 to kill their AA xp gain for the average improvement I would bring them. I doubt I could help the group kill 4 times faster, which is about the about of AA xp I would be taking from them.
The point is, in the end, Everquest needs to be more open then ever. Player numbers are down, there are more instanced zones (which means less casual PUGs), and in general there is a more entrenched player base (people who have been playing years and have a solid guild / group of friends). Basically it is more difficult than ever ... in my opinion ... to get a group these days. And by having a design decision that explicitly states "if you group with someone outside your tight level range, your AA xp gain will be negatively affected", is, again in my opinion, a Very Bad Thing(tm).