So, if you have a bunch of those...do you need the hundred million $+ stealth planes that Boeing builds?
Yes.
You can only target the stuff you know about. There's some new passive detection systems out there that will still find and jack-up aircraft mighty hard. So you lob a swarm of these microwave missiles to take out the enemy AA and communications systems you know about, and then send in the stealth craft in case there's still some functional threats hiding out.
Also stealth really means reduced detection range, not completely invisible, if you were to fly a B2 directly over an SA-3 battery at a linear distance of less than a mile to the radar system, it's still probably going to see it for a moment.