It's not always the speakers; sometimes it's the poor acoustics. In a subway station, for example, you have all those hard surfaces that sound waves bounce off of, frequently making the speech unintelligible.
Cheapo PA speakers can definitely sound mighty crummy. But many times, the root cause of poor vocal intelligibility can also be low quality microphones and/or electronics, or improper equipment settings. Of course, room acoustics are always a huge factor. And, as Nymeria noted, a mediocre PA system in a room with excellent acoustical properties will nearly always sound better than a quality PA in a bad sounding room.