- It's easier to see which one is active (i.e. the one you're editing), especially with the handle options that most people use.
- Thickness + opacity combined are much stronger than just opacity alone
- Perhaps it's less visually accurate - harder to tell exactly where value lies with regard to the curve line. This is a major reason I've avoided doing this before, though I suspect that it's not likely to be that much of an issue in practice (i.e. people will zoom in several times before they need more precision)