You've simplified things to an impossible extent to show that in an artificially constructed situation how the powers could be comparable. I don't find that to be a convincing argument of anything.
How are his assumptions/simplifications impossible? He starts with the assumption that the fighter will be next to 2 enemies at some point in the fight? That seems downright likely to me.
Assuming that you have a 50% hit chance? That may be arbitrary, but it certainly doesn't shift the balance in favor of one power or another, Sweeping Blow will get a bonus to hit in any case.
He starts out with a very reasonable default where Rain of Blows is 3.5 points better. So I don't get your problem with his comparison. Sure the other situations are less likely, but they WILL happen from time to time and that makes Sweeping blow even closer in comparison than the 3.5 difference would suggest.
There are a few other underlying reasons why Sweeping Blow is a fair balance with Rain of Blows. Some of them have been mentioned above, but I will summarize.
1) Rain of Blows requires an inferior weapon while Sweeping blow can use the most common (i.e. better) Heavy Blades - Greatsword and Longsword. Sure the lesser weapon is better here, but this is only 1 turn per fight and that lesser weapon will be used on every attack you make.
2) Rain of Blows requires a secondary (at best) stat to be unusually high, Dex 15 for a fighter.
3) Rain of blows loses a lot of steam if you accidently (?) kill the primary target with either the first primary or the first secondary, because you will lose the other primary and secondary attack. In other words, this power will sometimes get shortchanged by doing it's job (?) and should NEVER used against a minion and unfortunately, Minions don't come with a posted sign, so we usually have to guess.
4) Sweeping Blow is a Close Burst attack, one of only 3 that Fighters get in the heroic tier. There are many reason to want at least 1 close attack in your repitoir and those reasons usually collect in swarms and are particularly nasty to fight. Admittedly, this is the least reason, but after reading the stats for swarms, I have bad dreams of fighting them.
...It has the early Ranger melee encounter powers beat hands down and it totally puts to shame every single early Rogue encounter power.
Powers for one class cannot be taken out of context for that class to compare. Of course this power is better than Rogue powers of the same level - Rogue powers are generally weaker than even other striker powers because the Rogue feature for striker damage is SO much better. Better here + worse there = Balance.
...and might end up just popping a bunch of minions anyway.
Popping minions? That's really important! Those bastards hurt a lot more than they have a right to and getting rid of a handful saves on a lot of pain and eventual healing powers.
Clearly these powers aren't ideal for the same situation, but I think they're quite balanced. Particularly considering that with sweeping Blow you just have to be a fighter with a Longsword to use the power to its fullest and Rain of Blows requires a stat, and a weapon (or 2 feats) that you might not have otherwise taken. That is the picture of Balance to me. Yeah you can do better with Rain of blows, but my regular fighter is going to be doing almost as good with this power and a little better all the rest of the time. that = balance to me.