A horde of 50-60 Halberdiers with a Warrior Priest is a good thing, but there are plenty of things it won't work very well against. What it does do well is address the things that threaten one of the other effective units in the Empire army - Knights.
A unit of 12-18 Knights with great weapons or IC Knights with lances, led by a Warrior Priest or Arch Lector tends to outfight most things. It's extremely sturdy and deals out quite a lot of pain. However, there are some things that have them on toast - White Lions and Bloodletters in particular. Without a spell like Soulblight or Enfeebling Foe to slow them down, those units will make a right mess of the knights. That is where the Halberdiers come in - they are ideal for weathering a horrible wave of attacks, then fighting back at full effectiveness. Against the sorts of units I am talking about, after 1 good round of attacks, your loss of hatred won't matter much - you will have killed so many that the enemy effectiveness will have gone through the floor, and you will both start fapping about uselessly from then on. If nothing else, the enemy is then reduced to a point where the Knights can clean them up.
A horde of Halberdiers is also great because it's a good target for basically any augment spell you can think of - they're so mediocre that they can do with a boost in all departments - WS, I, A, S, T, Armour... For a unit that is at least moderately effective in its own right (and laughably cheap), they're very useful.
I liked the idea of Greatswords, however in practice they tend to be a let-down. Against many things they die almost as quickly as Halberdiers, don't do much more damage (the WS difference is often irrelevant, so it's just the +1S), and cost twice the price. +1 strength is great, but being guaranteed to strike last and costing twice the price gives me pause for thought. I don't say they're a bad choice, however it feels too expensive to field them in a size that makes them serve the same purpose as the Halberdiers. I think at the moment their stand out due to their Stubborn, which is a bit wasted when you're fielding them in massive units (where Steadfast often comes in anyway if you're using 5pt troops).