If your pair of lowbies, can kill a same level elite mob in 30 seconds.
And they use a third of their mana/health in the process.

Two such elites might be doable.
But three will very likely kill your team.

With an 80 in the party, you effectively get 10% of the experience.
But the 80 can kill 25 mobs at once, in less time then the pair of toons kills a single elite.
That's why boosting is strong.

Non-elites is the same deal, except they're less per kill or set of kills.
So not that useful for boosting, unless its boosting via very quick quests.