The reason you would use a Full Attack action would be to concentrate multiple attacks on the most dangerous foe facing you, rather than just one attack per opponent.
Rolling once vs rolling multiple times is not, IMO, what makes this feat overpowered as written - what does that is the potential number of opponents
you could damage, not the number of times a die is rolled. As far as I'm concerned, you should have to be able to take multiple attacks in order to damage more than one opponent in melee combat.
As for the "one-roll"/"many-roll" question, the language is unclear and can support either interpretation equally. In the absence of an official ruling (and possibly even in the presence of one..
), I'll go with the "many-roll" interpretation.
In the "one-roll" scenario, you could get a spectacular roll and hit all opponents just as easily as you could get a bad one and miss them all. As a general rule-of-thumb, most GMs (at least the ones I know) use a single set of stats for the "mook hordes" for easy bookkeeping. So, using the "one-roll" method, if you've hit one, you've most likely hit them all.
With the "many-roll" method, the law of averages takes over and you get a more even result - you *could* hit them all or miss them all, but it's more likely you'll hit some and miss some.
For myself, I prefer the more even, "law-of-averages" approach, but I can see where the "all-or-nothing" may be appealing, especially in the hard-and-fast combat of Conan. YMMV...
(And now I'll get off my soapbox, as I'm sure other board readers are getting tired of my ranting on the subject... 8) )