When you hire henchmen, they typically take a defined share of treasure and experience. Thus they make the PCs level more slowly, due to having the divide the experience over more party members.
What about mercenaries? There are rules for giving them experience AFTER they've been promoted to henchman (well, level-0 henchmen!), but if you just use them as standard mercenaries during wilderness encounters, should they still claim a portion of the experience?
The Domains at War rules suggest that once you hit mass combat scales, all the troops combined should be taking half of the experience from plunder. But I'm not sure how that scales down to a party with a half-dozen mercenaries or so.