PA now uses a "glitch-free" playback model which allows dynamic adjusting of the latency (and thus CPU load) to what the currently connected clients require without any discontinuities in audio. That's why PA is (or "can be" in a good setup) more power friendly than JACK is.
JACK is optimized for lowest latencies, CPU load comes second, and dynamic interruption-free changing of these parameters is not required. For PA however, we want dynamic latencies, generally low CPU load and the parameters need to be flexible.
But even if you would add "glitch-free" to JACK (which makes no real sense), it still is not designed for desktop use. PA carefully makes sure not to convert (or not even touch) audio data if not necessary. OTOH JACK only knows FLOAT samples, and thus audio type conversion is necessary usually at output and often enough on input too. That makes a lot sense for them. It's a very good choice, but it's not the right choice for the desktop.
The requirements of desktop/embedded use, and pro audio use are different. This results in different products. Trying to marry them is a great idea, but also very difficult to achieve. Of course, we should be trying to do this, but we have much bigger problems to fix before we start with that.