Well, when pregnant with DS2, DS1 was a horrible sleeper.
I spent many nights sleeping on DS1's bedroom floor, hand through cot bars. I actually had enough duvets and pillows living in the room to make it comfortable. Luckily he got better at sleeping before I got too big (as in went back to sleep in less than 2 hrs)
DS2 arrived, and I had crib in spare room, along with a double bed. I shared / co-slept (depending on the night) with DS2, feeding on demand. DS1 had the baby monitor resurrected, and it was wired to out bedroom for DH to deal with DS1. I suspect he dealt with 75% of the wakes, but there were still times when I woke for him first.
DS2 is now in a bed, but we have kept the double in there. If he is having a bad night, one of us (me if after DH has come to bed, DH is its before about midnight that he gets very restless) sleeps in the double, with him either in his bed or the double.
DS1 is now much better at staying in his bed, but if ill, tends to cosleep with me. DH is away for the next 2 weeks. I'm fully expecting to share my bed with a wiggly 4 yr old.
If either DH or I are ill and either not sleeping well, or disturbing the other, one of us will decamp into DS2's double.
So, musical beds here, and has always been.