At our hospital, I think the only unit with only 12 hour shifts is ICU. The rest of the hospital has a variety of 8's and 12's on each unit.
On our 65-bed Med-Surg where I work 3-11 (1445-2315) shift, the 12 hour nurses- 7A to 7P- never change their assignments at 3P, but may have one or two patients added due to less staff on 3-11. If they have made assignments in the morning without looking ahead to 3-11, the full-time, 3-11 shift 8 hour nurses often have to take a new assignment from the previous evening since the 12 hour nurses (7A-7P) refuse to change patients for 4 hours, which I agree with, for the sake of continuity of care. But, it HAS sometimes caused hard feelings between the full-time 12 hour and the full-time 8 hour nurses.
I work part-time so we part-timers come in already knowing and accepting that we won't necessarily have the same assignment 2 nights in a row; I think having the mixture of 8's and 12's is hardest on the full-time 3-11 shift 8 hour nurses. But also, the 7P to 7A nurses often have to change their assignments due to us 8 hour nurses taking their patients from a previous night, so they've been upset at times... It can really be a scheduling nightmare and I know everyone tries to be fair, but it can be very difficult with the mixture of shifts.
For the casual (or sometimes called, pool) worker, the 12 hour shifts make it possible for them to work for just 4 hours here and there when there are needs, which works well for those with childcare problems, to come in 3-7P or 7P-11P, and we can always use their help!