Since you asked...
For social I dance with beginners, and I think it is a good skill to adapt your lead to the followers ability. It certainly shows who is a "really good" lead if he can adapt to all levels.
For a lead to get better, you need to dance with great follows...
Better, being, loving the dance, taking moves past the "just moves" stage and doing those fabby moves, taking into account the music, that you could not put on a beginner.
If I wanted to show off, or just have a lot of fun... I would go for the most advanced follow.
I still love dancing with beginners

- but that will never make me push myself, in my own dance ability.