I don't know how much sense has got to write on a post of 4 years ago. Anyway, seen that the official answer still hasn't come, I give my opinion too.
Sweep is one of the funniest, maybe, but surely irrealistic game effects. It's something frightening and innatural. I appreciated the fact that very few units have got it. I use the untamed, and I can perfectly understand the disapointment in seeing the cursed able to sweep and the heartguard unable to do it, even if the warriors are alive, strong, cool and with great axes.
But you can easily solve the frustration playing the shadow tribe with an unholy champion, a line of 12 unlivings, and a warband of cursed as allied. Boosted by the unholy and sweeping everything.
I agree with the ones who think that "sweep" has to be considered a sort of ability of the troop rather than a charachteristic of the weapon.
In this second case, infact, a two-handed sword should be unable to sweep, as Wedge instead suggested, 'cause the sweeping effect is not due (in my opinion) to the cutting effect of a blade, but to the kinetik push of the weighted head of the weapon. That's why I could better imagine that a two-handed flail should have the sweep and a two-handed sword shouldn't.
In any case, I'm not Mr. Talamini nor a member of "dark simmetry" staff, so I think that, if they imagined a certain point cost for a unit they considered the effects of the sweep in this cost, and the addiction of a sweep to a unit who hasn't it should vary the cost of the unit of many points.
When I played my untamed in the first edition, my opponents always refused to use the climatic rules. After a lot of matches lost, I noticed my friends how my basic warriors had a PC very high if compared with their's, and we decided to reduce the PC of my tribal warriors of 3 points for every climatic ability they got, because, never using them, there was a value of unuseful ability which, on 1500 army points, led me to have 2/3 of the minis of my opponents on the field. PC of the units has it's sense because it considers everything of the unit's possibility.
Sweep is a very powerful effect. If you think that an elven axeman will rarely have the possibility to use it, well... maybe that's why he got it! The bone golem has it and i think it's a little exaggerated. Just really think to a Black sister champion with a sweep ability. In one very very lucky turn she could killl, with 15 in close combat skill (17 if considering the charge) and the secondary attack, up to 7-8 opponents not too much tough. I think that's what it happens when trying to justify the unreal with the "epic flavour", exageration leads from the nice to the ridiculous. With no offense.
Cheers to you all.