About Season 4...

I guess I'll answer. (If you're getting so impatient, lol)

I think that the girls didn't let Nebula and her army of females get on the Wizards first, because that would be taking their "glory" away from the show. The girls are always the ones to deal with - and ultimately finish off the seasons villain.

I think the writers thought making the Earth fairies take care of them (Which they could have done a long time ago) would take away all the "bravery" and "leadership" shown by the Winx whenever they battle.

In the end though, Nebula did help in the ultimate defeat of the Wizards, but it wasn't until the very last few minutes of the battle, when the Winx themselves were worn out. Like you, I too find it unfair that they couldn't get the revenge upon them [The Wizards] like they deserved.