There are several reasons to use PA instead of Jack as the primary sound server:
1: CPU/power usage. Jack uses a lot more CPU time than Pulse does.
2: Resampling. Pulse does it when necessary, Jack doesn't.
3: Per-Application volume control. Pulse can do stuff like this, jack can't easily (you can put in plugins, but they add latency and cpu overhead, also they would be a lot of work for end users)
4: Ease of configuration. Pulse pretty much sets itself up, jack takes a while to configure for low latency.
5: Ability to deal with jitter/high cpu usage. Jack gets buffer under-runs quite a bit as you push down the latency and boost the cpu usage, Pulse handles these situations much more gracefully.
As a side note, 25ms is still well within the acceptable latency range, I run around that on synths/guitar/other musical stuff and it doesn't feel laggy. 5ms is pretty much impossible on standard HDA stuff (you will get lots of buffer under-runs)