Well the new Dragon we're getting is mostly designed to deal with horde units so we may shortly aquire a new tool to deal with mobs.
Also a note that if the enemy has their horde more spread out, thus denying your Decimators their many attack count you can always use your Retributors.
For example if we have two units of 5 Decimators and Retributors (with Prime included) accordingly and we assume they attack a large unit that has their models a bit more spread out (imagine the first "rank" with the second rank to be almost 1 inch apart).
Now let's assume that with this spreading of the unit (and have in mind that unless the enemy player who knows our army and sees the Decimators will most likely apply this tactic) that each of the Decimators gains 4 attacks with the Prime getting 5 (since you'll most likely have him in the middle). Now with my rough math-hammer the Decimators will score:
Decimators: 4*4=16 attacks, 11 hit, 8 wound
Decimator Prime: 5 attacks, 3 hit, 3 wound
All in all 11 wounds. If the enemy has 4+ save (unlikely for a horde but a Mystic Shield may as well be in effect ) about 7 to 8 wound will be scored.
With 5+ save 9 wounds will be scored.
Now let's see the Retributors...
11 attacks, 8 hit and 2 out of them are 6s so 4 wounds from there, out of the other 6 hits 4 wound.
With a 4+ save 3 (turning to 6 due to damage 2 of the weapons) of them are scored plus the 4 mortal wounds we get a sum of 10 wounds scored.
With a 5+ save 12 wounds will be scored.
The big difference though is that your Retributors can face any threat be it monster or hero or other elite unit with much more efficiency than your Decimators.
I know my statistics are not 100% accurate, but I think you get the idea...