I second (third, fourth, fifth, ..) that a onesie or compression cover helps in covering up or reducing the appearance of a diaper but does not reduce leaking. I wear onesies for one major reason - to prevent anyone from seeing what I don't want them to see. I do get wicking at times when I push past the point of needing a change out.
Also, wearing an over tight (body suite like) reduces the pouch (has been mentioned) or droopiness of the diaper is a catch basin for all sorts of stuff. However, it causes more leaking.
For me, the onesie reduces the effectiveness to the benefit of not having a noticeable (psychological or actual) rear end. Even after years of having to wear protection, I'm still aware and self loathing at times of having to be diapered.