She doesn't actually need them to fight, it's about who had the most nukes (dragons).

At the end of the last episode Ulf lured Aemond out of King's Landing and to Dragonstone where Aemond immediately noticed new dragons and smartly retreated. That's a potential 6 v 1 battle (Syrax, Vermax, Moondancer, Seasmoke, Silverwing, & Vermithor), plus Daemon is a wildcard.

Vhagar is strong, but 6 v 1? That's a risk. Even if the riders are inexperienced (and die), those dragons are fighters.

And what backup does he have? Sunfire is nearly dead. His old town brother/dragon are far away. That leaves Dreamfyre, and something tells me Helaena isn't much of a fighter.

Arguably this puts them in a stalemate position. More accurately however it means no one gets to use a dragon. This is great for Rhaneyra who is winning the war of propaganda and smallfolk. She can afford to wait it out.

I agree that this is also a risky move. However at the moment all these new dragonriders need to do is stay at Dragonstone. That's a pretty safe prospect at the moment, so staying loyal is easy at the moment. I expect that might change before/during/after next episode since war changes things.