It may be improbable, but it came close to happening several times during our game this past weekend. With Tumble and jump available it can happen a lot if the player wants it too.
Besides, I ruled it only applied to bonus' given by feats, not spells.
Tell you what, if you think it isn't over powered and useful, ask your respective DM's to hit your group with NPC's wielding range weapons, inside of a keep, where movement/space is limited. Give them the feats leading up to whirlwind, boost them up with prayer, bull strength, etc..., throw in the feats with bonus' for good measure.
If your PC's are still alive and you still think the feat isn't overpowered, keep allowing it.
As for the useage of "or" in a sentence, I looked it up in the "Holt Handbook on English Composition" and the "Harbrace College Handbook" before I posted my response. I'm satisfied my interpretation is legitimate, but I agree it could be looked at as Darthjaye wants too. The wonders of the english language, indeed.
Saviomagy,
Apparently, I have different experiences than you. Whirlwind deals out a lot more damage than Great Cleave, or Cleave. Plus, since the DM's I play under usually have us fighting balanced encounters, I only see Cleave used with any regularity.
See, when your playing 9th level characters you are fighting NPC's with 30+ HP's. No one normally does that kind of damage with any one hit, unless they crit multiple times in a row.
So I see a frequent amount of Cleave, but rarely Great Cleave.
Now look at Whirlwind, which granted has many very useful prerequisite feats. Your in the middle of your parties front line. You have a Spiked Chain weapon (using the character from my game this past weekend) which is enchanted as +1 Goblin Bane (they are fighting Eldritch Goblyns of Erde, and I am allowing them to be affected by the bane, despite the fact they are dwarven blooded in my game). So this character is getting +3 to hit and damage + 2d6 damage, he has an 18 STR for an additional +4 to hit and damage, he is boosted by Bull's Strength, taking his strength to 23 (+6).
So he is doing +9/+9 +2d6 +2d4 (chain damage) without feats or BAB. With BAB he is +18 to hit.
He has several Goblyns charge in, centered on him/his line. By using Whirlwind he can deal out an average of 19 points to every one of the Goblyns that come within his reach, not to mention the AoO's they provoke for passing through his threatened areas just to attack his companions. He is +18 to hit against an AC of 19 in this case. Darn good chances to pull off a hit on every one of the Goblyns, plus a couple of AoO's since the PC has combat reflexes.
Now throw in bonuses from prayer (average 20 Damage), how about bard song (average 22 Damage)? Now throw in bonus' from feats (average of 25+ with Specialization/power attack damage).
So for giving up his iterative attack he gets to do anywhere (on average) of 19 to 25+ points of damage, at least once, to every NPC that is within 10' of his character when he attacks. In comparison he would get to do 38 damage to only one creature by not doing his Whirlwind.
Then his fellow party members get to lay into the Goblyns, one of whom has great Cleave. Because the mighty Chain of Winds has done significant damage to each of the goblyns, the Great Cleaver actually gets to great cleave because she also has a Goblyn Bane weapon and has a high strength and is power attacking with +5.
So yes, I do know what I am talking about. I think most DM's would quickly tire of a game where the PC's easily mow down CR 4 and 5 and 6 and 7 (depending on actual HP's of the NPC/monsters) creatures because of whirlwind making Great Cleave happen even more often. I know I would, and will.
So if you think it isn't too powerful to deal out that much damage, and make cleave's and great cleave's that much more common for the other PC's, well, its your game. You can have the power level be whatever you want it to be.
So I'll keep my opinion that Whirlwind is very powerful, and gets worse with every bonus added. Not to mention how sweet the feat prerequisite's of Combat Expertise, Dodge, Mobility, and Spring Attack are on their own.