Nah man, nah. Prep reduces the next spell by 3, so assassinate would cost 2.
But yeah, bgh in miracle rogue was a thing before erf I'm pretty sure, then zoo and other aggro decks became more prominent so its a meta choice. But yeah it does fucking suck when shit like that happens, I had the exact same type of situation happen to me last season and the miracle rogue I faced in the few games I played handlock ran not one, but two fucking bgh when no one else was.
I don't think I'm going to push for legendary this season, been too busy playing other games and I've bought/am buying about a dozen games on the summer sale.