I don't really see a need for a phones jack on a mic preamp, but I do find it valuable to monitor from the recorder for some situations though certainly not all of them or every time. Getting things just right is more than just monitoring levels on a VU or peak meter. I've used highly isolating in-ear monitors to tweak critically spaced omni spread and height and small changes in both dimensions were quite audible in the field. I probably would have been happy with the recordings without doing so, but sometimes it's that 9th 10th that really nails it. I learned a lot by doing that, basically reducing the record > take it home & listen > record with a slight change feedback loop to an instantaneous, though less precise reiteration.
I don't do that regularly nearly as often as I used to now that I'm more familiar with what to expect from various changes and obviously sometimes it's not practical given the situation. As amateur live recordists we often find setups that work consistently, sound good and aren't too much of a hassle to set-up given the considerable constraints under which we do our thing. But monitoring can be a very valuable tool to check what the mics hear. Can you imagine a professional recordist making a purist stereo recording without monitoring the recording to tweak the mic setup? It's very challenging to get everything sounding just right even with monitors set up in an isolated remote room.
