This is a proper use of nets! The harpies get their wings tangled in the net when it falls on them, and the Argonauts have to rush in before they untangle themselves.
This would have been a great opportunity to bring back the healing flowers. It'd be a nice show of Jason learning from Medea and using that knowledge. A sign of respect. It would also let the movie reduce the powers of the fleece: it blesses the nation rather than individuals.
I too afford my enemies the respect of completing their rituals to summon unstoppable monsters when I could simply cut them down where they stand. It's just sportsmanlike!