It really depends. Most of the time, at higher levels, a fighter is better off using his iterative attacks to focus on one enemy at a time.
But against:
1) Low HP Opponents, who you can take out in fewer hits (not rounds) than there are enemies (2 hits each on 3 enemies) WWA will end a fight faster than focusing on one at a time.
2) Really high AC opponents: If your 3rd and subesquent iterative attacks are not likely to hit anyway, you might as well Whirlwind. Because all WWA are at your highest BAB, your second WWA is effectively at +5 to hit, your 3rd at +10, your 4th (if you have a 4th iterative attack, even) +15. When faced with two opponents where your first attack hits on a 5, your second on a 10 and third on a 15, it usually isn't a bad Idea to WWA and hit each of them once on a 5 -- instead of hittin one of them once or maybe twice.
3) Really low AC opponents in conjunction with power attack. If you only miss on a 1 with your highest BAB and can pump +5 or more into damage with Power Attack you might want to hit as often as possible. Heck if you have a +20 attack bonus against 2 opponents with an AC of 16, I would forego the 3rd iterative attack and even risk +7 to Power Attack and only hit on a 3 or better. Statistically it might not be perfect, but your dealing with large numbers by this point so you've got a bit of a margin for error.
4) You have allies who are focusing on individual enemies. Your WWA will wear down all of the enemies (especially in situations #2 and #3 above) more than would be possible without WWA. Your comrades will probably get the killing blows, but you'll be shortening the fight by a couple rounds if you use this tactic.
5) There is some magic involved that gives you a bonus to damage for only one or two rounds. You want to get the most out of it so you use WWA (at your highest BAB) to ensure that you hit as many opponents as possible..