Alright, first off I want to apologize for my absurd post length. I got a bit caught up in writing.
Basically, I'm trying to lay out the scenario where Bag of Rats causes problems, which is in dealing with powers that target creatures. It is my opinion that powers that target creatures do 2 things: allow flexibility in how a power is used, and/or provide greater tactical difficulties in successfully employing them. The Bag of Rats shorthand causes problems with these powers.
What about close or area attacks that target "creatures" and have a hit or effect line? The Bag of Rats ruling states
"When a power has an effect that occurs upon hitting, missing, or otherwise affecting a target, the effect takes place only if the target in question is a meaningful threat."
Would those not target allies because they "are not a meaningful threat"? It seems a straight forward application of BoR leads to the conclusion that any power that targets "creatures" instead targets "enemies", since neither you nor your allies are threats and thus cannot be the target of powers that have "Hit:" or "Effect:" blocks.
Assuming you disagree with the above, how do you handle situations where you target a mixed group of allies and enemies with such a power? If you miss all the enemies and hit the allies, would you allow the hit/effect block to take place? What if you missed you allies and hit the enemies? What if you missed everyone?
To illustrate the above questions, take the Warlock level 3 encounter power, Otherwind Stride:
Standard Action Close burst 1
Target: Each creature in burst
Attack: Cha vs. Fort
Hit: 1d8 + Cha damage, and the target is immobilized until the end of your next turn.
Effect: You teleport 5 squares.
Fey Pact: You teleport a number of squares equal to 5 + your Intelligence modifier.
Assume there are only enemies in the burst when this ability is used. I think no one would argue that the ability should resolve normally, with attack rolls resolving and the warlock teleporting.
Now, assume there is one enemy and one ally in the burst when this ability is used. Even better, say the warlock really wants to use the teleport to hop across a chasm and grab the McGuffin. Would you allow this ability to be used?
- Can the ally even be targeted by this power, even though s/he is not a "meaningful threat"? Or are both the enemy and ally targeted?
- What if the power hits the enemy and misses the ally? Is this acceptable? The warlock used a power, targeted an enemy, and gets his teleport.
- Do you consider it acceptable if it misses the enemy, hits the ally, and the warlock gets to teleport? The warlock used a power targeting an ally, and got to teleport.
- What if it misses everyone? The warlock gets a 'free' teleport!
- What if it hits everyone?
Continuing on, the warlock doesn't care about the possible damage, he wants to be able to hop the chasm and grab the McGuffin to save the day. Assuming he's still next to one enemy and one ally, how do you handle the following:
- The warlock misses everyone because he had a -2 to all attacks imposed by another enemy. Would he still get the teleport?
- The warlock misses everyone because he had a -2 to all attacks due to voluntarily moving through a terrain hazard earlier in the encounter. Does he get the teleport?
- The warlock misses everyone because he had a -2 to all attacks due to changing his implement. Does he get the teleport?
In the same vein:
- The enemy in burst uses an immediate interrupt to shift out of the close burst, such that the enemy is no longer a valid target. Is the ally still a valid target?
- What if the warlock is only surrounded by allies during combat?
- What if the warlock is only surrounded by allies out of combat, but just after a short rest (i.e. his encounter power is available)?
There are dozens of powers that target creatures and have hit or effect lines that spawn these questions. I think it is difficult to draw a delineating line with the strict Bag of Rats ruling in these cases (i.e. stating "you can only use your power when an enemy is in the area of effect" leads to nonsensical situations like "Well, the enemy shifted out of the effect as an interrupt, and now only allies are in the area of effect, so sorry, you can't teleport, Mr. Warlock!")
However, if you drop the whole "allies aren't a meaningful threat" routine, all of the above questions roughly boil down to "Yes, Mr. Warlock, do your thing." I have found that in general, this is not terribly game-breaking.
Even if the warlock was to expend his encounter power to get a free teleport with no enemies in sight, I have no problem with it. Once every 5 minutes, he can teleport 25+ feet. Sounds good to me for a warlock who's shtick is being able to take shortcuts through passages between the realms. Even better, the twisting of his passage will hurt bystanders, so he must choose his strides with care.
In combat, he teleport AND damage enemies, or teleport AND damage allies (or some mix of those two), or teleport AND do no damage. Any of these could be done at the cost of a standard action and the expenditure of a valuable combat resource. The last two options are definitely less beneficial than the first non-disputed option, so what's the harm in letting him take a action that gives less benefit than that actions normal application?
Now, I know this is one specific example, but I believe that it is representative of the general case. I would like to see cases in which allowing yourself and/or allies to be "meaningful threats" results in game-breaking balance issues.
The only issue I can really come up with is at-will surgeless healing on a hit or effect, but that seems more of a problem with at-will surgeless healing in general than a target definition problem.