Shares of Joby Aviation (JOBY +0.73%) trade below $7 as of this writing, near their 52-week low, having lost about two-thirds of their value from a 52-week high of nearly $20. Investors, it seems, may be tired of waiting for electric air taxis to turn into meaningful revenue.The company, meanwhile, isn't waiting. On Aug. 11, Joby announced an agreement to acquire Resonant Sciences, a defense technology company, for about $500 million -- about $450 million in cash plus $50 million in stock. It's a purchase b ...