This is true, but with Defender you have to choose between protection and doing damage. The moment you exit the bubble, you become vulnerable to hostile fire. Ignoring bosses, tethers will subdue enemies while you shoot them with your weapons, meaning you don't have to choose between safety and attacking. Nightstalkers can also generate quite a lot more Orbs of Light than a Defender bubble, meaning your allies can charge their Supers much more quickly. Both subclasses have their strengths and weaknesses, and skilled players in either of them will be a huge help to their team. That's why I say they're tied for top spot in PvE.