I'm trying out spears at the moment. My reasoning is that this provides a lot of flexibility. Depending on whether you want ranks or damage output, you can mix between horde, 8-wide, 6-wide or 5-wide. Spears allow you to maintain a fairly decent damage ratio no matter what your formation. Its an extra 25% damage increase over HW&S when 5 wide, for instance, whilst the parry only saves 16% more (although there are occasions where you wouldn't get any save, but still keep the parry).
I wouldn't have paid the points for it before, but as a free upgrade its worth it. Also spear saurus in horde look really cool - there's just things that you can do with spear weapons to make the unit look much more dynamic.
If you're looking at 24 or under then I probably wouldn't bother, as you're likely to lose the models before they get to attack with their spears. At 25 or over, though, I'd seriously consider it.