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.
I had always felt that it was not a wizard's role to deal damage, but to control the battlefield and stymie the foes so that the key damage dealers can attack relatively unmolested. It is nice that their spells also do some damage, but generally, I try not to depend on that. For example, I would never choose damage-only spells like fireball. Status-effect spells like icy terrain, stinking cloud and sleep are where their true strength lies, IMO.
The damage adds up, yes, but way too slowly, IMO. Each round you do not take down an enemy is an extra round it can contribute in battle against your party. Which may result in the expenditure of additional resources such as healing surges and restorative abilities, because while your party may be dealing more damage, they are also taking more damage as a result of the foes having more actions available to them.
While rain of blows alone may not be enough to 1-shot a foe, it should be able to still contribute a decent amount of damage, which when combined with the attacks from the rest of the other players (such as the strikers), might just be enough to push the foe over the edge. Moreso than sweeping strike.
The combined damage dealt by sweeping blow will have to massively outstrip the overall damage dealt by rain of blows for it to be considered worthwhile (IMO, by at least double). And I view this sort of scenario more an exception than the norm, in that the DM would rarely ever set up this sort of situation. At least, that was my experience with whirlwind in 3e...

As has already been illustrated in this thread, Rain of Blows does not necessarily vastly outstrip the damage that Sweeping Blow does.
In terms of total damage dealt, I agree. In terms of the quality of the damage done, I have to disagree.
I still stand by my earlier point that rain of blows generally remains vastly superior compared to sweeping blow, by virtue of it allowing you to concentrate all the damage on a single foe, rather than forcing you to spread it out evenly. Under normal circumstances, I am not going to slowly chip away at a horde of foes and attrition hp with them, because I am sure to come out for the worse (enemies' hp outstrip that of a defender's by too great a margin to make this a worthwhile venture, IMO). So I feel that it is still better to take down any one foe ASAP and prevent it from contributing to combat any further, before moving on to the 2nd target, and working on each one of them in a systematic fashion, until your party is the only one left standing.
If I am going to deal damage to it, then I should go all the way and make sure every bit counts and works towards reducing its hp to zero ASAP. Anything less is simply unacceptable, simply because as mentioned earlier, they are every bit as efficacious in combat regardless of whether they are at 1hp or full hp.
I also find it funny that there are certain powers which appear to benefit other classes more than the original class itself. For instance, I feel it odd that rain of blows seems to mesh better with a brutal scoundrel than a fighter. But I am not sure what to make of this trend...

Just my 2 cents.
