Yes,  the bushings are dampeners and would get torn apart over time without them.  As you tighten the shock mount bolts the bushings get squeezed a bit by the washers on either side and tighten up, they would not allow for the small amount of rotational movement generated by the suspension travel at the upper and lower mount studs.  If you don't put a spot of grease in there, either the sleeves or the mount studs or both will wear.

14mm inner diameter sleeve. That's 9/16 inch in American, seeing as you're in 'Bammie...

And how low? Mobile? Dauphin Island? Bayou Labattre (can't remember the spelling, but have family down there being displaced by Vietnamese shrimpers)...