They are only there for regulatory reasons and not really intended for serious use. If they were, we would be expected to practice with them. In ten years, I have tried flying the sim once on the standby instruments and that was only because we had some spare time and I insisted.
Australian regulations require testing of competency on standby flight instruments as part of an instrument rating tests. It is quite tricky in the 737 Classics since the stab trim has to operated manually by means of the big wheel. The copilot has no standby flight instruments although I think he has a clock if he is lucky. It is certainly a challenging exercise. If you do not have the demonstrated skill to fly on standby flight instruments to within regulator tolerances then you don't have a job because you will not hold an instrument rating.
Reminds me of another era when undergoing instrument rating tests in a RAAF Dakota where part of the test included conducting single pilot, asymmetric, aurul null NDB let-down, on limited panel (meaning no AH and no DG but you had a tiny E2 type compass held in place with two bungees between the pilots windcreens)
I must say, that required real instrument flying skill. But the satisfaction gained from being able to hack it was inestimable. What a difference between the knowledge in those far off days you could truly fly on instruments, to today's generation when a manually flown visual approach on a sunny day is beyond the capabilities of some airline pilots who are so automatics dependent. Comparing apples and oranges? Well, maybe. But I know who I would trust when the chips were down on a dark and stormy night.