Concerning progenitor’s stats, I never said they were bad, just not amazing. 5/4’s can kill more things than 4/5’s, but 4/5’s have many more combinations of damage sources they can survive. If I could run my thunderhorns as 5/4’s instead of 4/5’s, I’d say no. If I could run them as 6/4’s instead of 4/5’s, I’d have to think about it.
Concerning progenitor’s ability,
This is my personal preference. I’m very leery of using any cards that need any kind of established board to work well, because of how many ways there are in this friggin’ game to nix the other guy’s stuff. Progenitor’s ability has great potential, it’s just not my style. In a less removal-happy game than Duelyst, I’d definitely consider it.
And it is true that I didn’t think about cloning multiple small things, rather than one big thing, so that’s my bad. If you can go young silithar (or other 2/3), ragebinder (or other good 3-drop), progenitor, that puts your opponent in a bad spot. Assuming they kill the ragebinder egg, you still get a 5/4, a young silithar and absorption of damage/removal out of your four-mana card, so it can definitely do work.