There seems to be quite an emphasis on low latency in this discussion. When recording, the issue of latency can be side stepped by monitoring direct from the mixing desk or microphone preamp, or if your interface permits, low latency monitoring from its internal hardware. These methods aren't always available, but in a pro recording situation they often are.
The most important reason for realtime scheduling privileges IMO as an audio engineer is to ensure that nothing gets in the way of audio throughput or processing. Just increasing buffer size is not an option beyond a certain point, as it is limited by the interface hardware. JACK was designed for pro audio (the fact that hobbyists and non audio engineers also use it is a side issue) and needs to perform reliably enough for those of us whose jobs depend on it.