Against hords according to the rules one does DoS/2 hits. Is Swift/Lightning Attack against hordes completely useless?

 No. You hit normally as per lightning or swift attack plus 1 hit/2DoS. 

Amaimon said:

You get 1 hit per 2 DoS on a WS test vs a horde. If that WS test happens to be a Lightning Attack you can potentially get 1 hit for every DoS (read: for every hit)  that does damage. If that weapons is a power weapon, you get 1 additional hit for every of those hits with lightning attack.

An example:

You have Lightning Attack and a Power Sword, you roll 10 on a vs 60 WS test on your Lightning Attack for a total of 6 DoS.

You get a hit for 1 DoS, 3 DoS and 5 DoS; a total of 3 hits from your WS test.

You get a total of 6 hits from Lightning Attack (assuming your WS Bonus reaches that high). If each of those 6 Lightning Attack hits deal any amount of damage to the horde, that's 6 magnitude hits as well. As you dealt the damage with a weapon with the Power Field quality, those hit deals an additional 1 magnitude damage, for a total of 12 hits.

So all together you did 3 + 12 = 15 magnitude damage with your attack.

Thank you very much for the elaborate example!

He's wrong about power field. Weapons with power field quality inflict one additional hit only. ever

Page 349, Melee : Melee weapons with the Power Field Quality inflict one additional hit.  

It's also on the GM screen, which is useless besides that.