I'll just chip in my experience here. When I first started riding I spent too much time out of the saddle and would get tired. Then I learned about spinning and started spinning my way up hills. Then I became glued to the saddle too.

Over the winter, using my trainer and spinervals I began to reach a balance. Thus I agree with those who suggest spinning classes or riding on a trainer. I would add in that the spinerval, "Uphill Grind" was the one that helped me in this instance.

I think there are several reasons to come up out of the saddle. You may need an additional bit of power to finish a hill or start out at an uphill bit of intersection (esp if you are in the wrong gear for that last). It gives you a different position to rest your seat, back, neck, whatever. If you end up in a dangerous spot on the road, it gives the additional power to pedal out of it quickly no matter what gear you are in. Oh, & it's fun.

Your stops and starts will be easier too.