Does never using the easy front gear give a better workout?
It is not necessarily "good" for your workout. If you drop into the 'easy' gear, you obviously have to pedal faster to maintain your speed. Going at a slower cadence in a higher gear will just trash your knees after awhile.

Select the gear that yields a similar cadence as when you are riding on the flats, while maintaining a similar energy expenditure.

Your chain rubbing problems should be solved by adjusting your derailuer and properly maintaining your bike.