Guild Wars 2

Does it affect only regen rate or does it also apply to sources where you get a chunk of it, say the 50 endurance on steal?

Weakness reduces your target’s damage, and your target’s endurance regeneration rate. It drops both by 50%.

They won’t hurt as much, and they won’t be able to dodge as much.

I do not think it applies to Steal. If you are afflicted by Weakness and you steal, you get 50 endurance. period. If it mentioned endurance regeneration, it would have have had a an impact, but not in this case.