I second this. I plop and pixiecurl diffuse everyday and my hair is definitely less curly if I skip either step (though I will occasionally skip one or the other--most likely diffusing--if it's a lazy day and I'm planning to stay in). If I skip both, my hair is not very curly. Just random wavy bends and LOTS of frizz.
To be honest, as much as the PJ in me wants to believe that there's a miracle curl-enhancing product out there just waiting to be discovered, I think that plopping and diffusing properly do way more for my hair than any one product ever did.
2b-ish--embracing my waves this summer
Aussie Catch the Wave condish, L'Oreal NutriGloss condish; Nexxus Designing Texxtur Cream, Rusk Radical Creme, Pantene Curl Defining Mousse, Paul Mitchell Sculpting Foam; coconut oil for DTs