Edit:
I agree with this . . .
Original Post:
Make a 10% XP Penalty Per Hireling
This one is obvious.
DDO needs people to play with others.
This would help to nudge players in the right direction.
Also, keep hirelings stupid. Don't bother to improve them or make them customizable. Focus on other, more important things.
Lastly, establish DDO etiquette to dismiss hirelings, eg. healers and rogues, when their role is filled by a real player in the party. For example, it is rude to keep a healer hireling summoned when you have a real healer in the party.
The strange thing is I saw an lfm about a week or so ago saying that there *was* a penalty for hirelings. I have tested it though with a single hireling and there was no indication of a penalty in the progress panel after summoning a hireling.