But you neglected to mention the fact that for sweeping blow, the multiple attacks are spread out evenly amongst many enemies, while for rain of blows, they can be concentrated on a single enemy.
Under most normal circumstances, I would prefer to quickly focus fire on a single enemy and take each down ASAP, rather than slowly chip away at a horde of foes. In the former, an enemy that is killed that much more quickly cannot retaliate, and I have thus benefited by taking less damage in return.
I do not disagree with this as a general rule of thumb, but just because you get multiple attacks against a single foe does not mean that you will kill it (Rangers often fail to kill foes, even bloodied ones). Nor does it mean that a single attack against multiple foes cannot kill one or more of them either.
Wizards do this "chipping at multiple foes" all of the time and the damage seriously adds up.
Basically, if you compare all the 3rd lv encounter powers a fighter gets, rain of blow's damage output vastly outstrips what the rest are capable of. Maybe not broken, but it still comes across as still being extremely strong.
As has already been illustrated in this thread, Rain of Blows does not necessarily
vastly outstrip the damage that Sweeping Blow does.
It only does so with the proper (typically somewhat inferior) type of weapon and only for Fighters with a Dex of 15 and only in situations where the Fighter has 1 foe on him.
Once he gets to 2 foes (and the rest of these conditions apply), Rain of Blows only does a small amount of damage more on average then Sweeping Blow (~17 points of average damage per round does not vastly outstrip ~14 points of average damage per round as per the example above).
For many Fighter designs, Sweeping Blow averages more damage. The player really has to design his Fighter with the proper Dex and the proper type of weapon for Rain of Blows to gain the slight edge.
At least in our games, melee types tend to try to hold a corridor or hold a doorway or even hold a corner of a room, both tactically and with Marks. In those cases, it's real easy to have 2 (or more) adjacent foes because more monsters in 4E are melee foes.
Solo opponents are the exception, not the rule.