Timothius:
What my brother and I have done when "allies" are recruited is offer that, to prevent combat from slowing down, (since every new unit in initiative makes a longer combat) roleplay out the new ally's turn instead of doing it mechanically. This does two things:
1) it takes the mechanics out of the equation.[/quote]
Interesting. I'm not all that concerned about the mechanics, or slow combat in general. I simply don't want combat to become significantly easier.
But I'd like to understand this: does what you describe have an effect on the game, like extra damage to a monster, or is it just description, like in a movie where extras might attack but only the star really has an effect?
<quote Timothius>2) prevents loss of experience. Remember that every new ally means the xp gets split up even more.
I'm not sure that rule really exists in 4th Edition.
PC allies get a share of the experience, but NPC and monster allies might not, simply because the rules don't really address that.
But, now that you mention it, that does seem like a fair way to handle it. I could say that I would subtract the experience value of a monster ally from the experience gained in an encounter, on the basic assumption that a monster on one side cancels out a monster on the other side. Obviously, it's not that simple, but a reduction would seem reasonable.
Timothius:
Alternatively (or additionally), you can just add more enemies in future encounters. :)
I don't like doing that in general, because it feels like simply negating an advantage they created for themselves. But, if I followed your advice and reduced their experience reward, they might seek out tougher encounters themselves, in order not to slow their advancement. Good thought.
I thought of another thing, but I'll put it in a different post.