I've heard this rationale before, and I still don't buy it. The barbarian has the same option to use all their dailies in one encounter as everyone else. Sure, a rage effect can potentially last an encounter, but that's a privilege, not an entitlement. It's a privilege because other classes don't have the option of milking a daily for an entire encounter.
If you use a second rage in the same encounter, it's not like you're giving up something and getting nothing in return. The old rage effect will end and the new one will take its place. There's no big shaft here.
The various rage powers don't actually do much damage on their own. Compare, for instance, the barbarian's 15th level thunderfury rage (doing 3[W] damage, half on a miss) to the fighter's dragon fangs (which gets two shots for 3[W], each doing half on miss and adding static modifiers twice) or serpent dance strike (getting 3 or four attacks for 2[W] and static modifiers, depending on how you read "repeat the attack up to three times"). And that's just fighters. Rangers have blade cascade and confounding arrows; rogues have slaying strike, vicious cooperation, and ripple effect. If, on the other hand, the barbarian uses rage strike, he gets 6[W] plus static modifiers by spending a 15th level power. This still leaves him outclassed by everyone, damage-wise (except maybe the rogue), since the fighter and the ranger will be adding their static damage multiple times.
Also, note that other classes can benefit from (some of) their dailies for an entire encounter. Wizards, warlocks, and clerics have sustained effects and powers like armor of agathys. Fighters, rangers, rogues, and warlords have stances and can inflict other long-term effects with powers like splintering shot, walking wounded, and lead the attack. Paladins even have a few powers like wrath of the gods.
Rages are more like stances than like sustained effects. But no martial character is obliged to take any stances, or to take more than one stance. Barbarians have no choice in the matter. So they have more need for a rage strike-type effect.
While you say spending all of one's resources in a fight is sometimes called for, I say wad-shooting happens far more often than it's called for. The "compensation" provided by Rage Strike is largely unwarranted--as it's based on a misplaced notion of "have your cake and eat it too" entitlement--but making it an encounter power at least makes for a reasonable middle ground.
Perhaps people expend all their resources when it isn't called for. That doesn't mean that expending all of one's resources is never called for, and hence the barbarian should be able to do so while still performing their role as strikers. And since rages often do less damage than the dailies of other classes, rage strike is a good way of letting the barbarian go all-out when called for without sacrificing their damage output.
Well, understand, I'm not really concerned with comparing 29th level powers in my early paragon-tier campaign, but even so you may still have a point. That's the part I'm willing to playtest out.
The example above is mid-paragon. It turns out, though, that generally a well-built fighter or ranger will do more damage with their dailies than a barbarian, even with rage strike, at more-or-less every level. At 9th level, the barbarian fares better, but even there the 5[W] rage strike is outclassed by attacks on the run for rangers, and provides comparable DPR against jackal strike and thicket of blades for the fighter.
