Though I'm no expert, I don't think horns are all that bad.

Pros:
- Psychological
- Additional protection against side blows to the head
- Rare instance of use as weapons
- National identifier (similar to the German spiked helmet)

Cons:
- Additional Cost
- Additional Weight
- Helmet is a larger target

As for the point of the horns being a disadvantage by catching weapons that would otherwise probably slide down, if they didn't catch it then the weapon would probably impact on the shoulders anyway.

Personally I think the sheer pyschological edge of horned helmets (especially against Christians) would make up for any faults, even more so if used by an elite unit.