Okay...
So far as I can tell, this arguement revolves around the nerfing of CLeave in specific situations...
1) the summoning creatures/bag o' puppies tactic:
Looks like that is pretty much up in the air. By the RAW, it is technically legal (dependant on your DM's definition of 'enemy'). From the higher powers stand point, it is dependant on the views of your deity (which goes back to the DM). I do have some questions on this, and please think beyond the RAW (since I have be given and fulfilled requests on making cinematic/real world sense of a written rule interpretation).
a) if a creature comes to your aid, is it really an enemy?
b) if a wizard attacks you with summoned creatures to gain the advantage, are you realy better off, especially if they DO hit (natural 20's are still out there).
c) if yur deity allows for such sacrifices or uses of their minions, can you give me an example of hitting an ally personally (with your own melee weapon) to gain some advantage?
Please be as specific as possible.
2) The targets did not know about the other.
Again, this is dependant on the DM, but IMHO, the question should be 'what did the fighter perceive, and how are they controlling the situation?'.
3) Reach weapons
Again, up in the air. I've seen (as have many of you out there) martail arts expos where practitioners used 10' Bo staves and chain whips to striking effect (yea, I know, by the RAW you can't Cleave with a whip, but a spiked chain works in a similar manner).
4)It doesn't seem fair
That's between the DM and the players. Personally, I'd much rather deal with the fighter being able to CLeave my party to heck and back, than worry about fair. More of a challenge that way
5) It doesn't make sense, cinematically, realistically, or visually.
For this, I would ask those with real experience of some kind (none of us have the experience to be 20th level fighters, but still...) to give us a situation that they have been in, and whether this tactic did or did not work. Granted, these accounts will be biased by perspective, but it should give an a decent overall view as to what works on average.
if such an example is given, please don't rebuttal with 'said person does not have the CLeave feat' or some other mechanical arguement. Given the abstract nature of game we really don't know if they do or don't. My point is to gain a series of examples, then use said info to see if the above examples (I'm behind a door, yet someone tripping behind the wall gets me killed) are or are not possible.