Interesting. I actually think, the Skiritai are actually one of the best (and most underestimated) units in 0ad, especially for new players.
Not in terms of stats. But they are available as soon as reaching phase two, nearly twice as cheap as full champion units, and they can gather resources, champions can't. When reaching phase 2 at 100 pop, you can easily train 100 Skiritai before hitting the 300 pop cap, and that while sacrificing barely any booming speed. And the Skiritai are significantly stronger than basic citizen soldiers, especially spearman. 100 of them so early can give a huge advantage.
Athenian marines on the other hand require a gymnasion to be built and then the tech to be researched, can only be trained at the gymnasion itself (while Skiritai can be at the barracks), and cannot gather resources or build anything. It's simply impossible to train them at the speed that you can Skiritai. Ive tried several times but never managed to take advantage of the fact that Athenian marines are available in phase 2 already, at least not without significant drops in booming efficiency.
All in all, Skiritai can't compete with champions, but can replace your much weaker basic spearman in huge numbers, which can give you a bigger advantage than a few (but individually stronger) champion units.