This coming from someone who hates Rainbow Dash. She's proven herself to be not only as good as, but better than any of the wonderbolts, including their captain.
She's the only pony in the world who's ever performed a sonic rainboom...multiple times even. The first time she did it, it earned her her cutie mark, which says A LOT about where her talents lie.
She saved 3 wonderbolts lives, including their captain, when they stupidly let themselves get knocked out by a flailing victim.
She single-handedly coached the pegasi of a tiny rural town into almost breaking the wingpower record set by Manehatten; a megalopolis; only failing due to an unfortunate featherflu outbreak.
In short she's proven herself a million times over. She should not only join the wonderbolts, she should lead it, because she's better than any of them. Exactly what more does Spitfire want her to prove?