Is Holy/Unholy Champion worth it as an ability?

Holy Champion and Unholy Champion are abilities that both cost 10 points and effectively increase hero's Attack and Damage by +2, which is a lot (two times more efficient than simply levelling up those stats).

However, they are also available as a spell that costs 25 mana to cast and 8 mana per turn to support. When playing a campaign, skill points are actually an a lot more rare resource than mana, as hero level is very limited. I typically leave skill points for much better things like Cold/Lightning Strike, Life Stealing, Parry, Charge etc.

Is it worth it to get permanent Champion ability using skill points?

It's a powerful effect against the right alignments, but as it can be replicated easily with a spell, it's not worth the skill points. There are however two situations, where you can consider it:

a) When you cannot cast the spell, but want the effect. E.g. on a single map game, where all your opponents are good aligned, but you don't have any death spheres to cast unholy champion.

b) In multiplayer. Human players are likely to use dispel to strip your heroes of buff spells, getting un/holy champion as a skill means it cannot be dispelled. The AI also sometimes uses dispel, but usually only to dispel waterwalk/fly, when your unit is over water.