Yup, what she said. Mostly in our case because they want to. They've both loved co-sleepin with DH and I. Admittedly, more than I love it. I love the cuddles but have huge difficulties actually sleeping with anyone touching me. It would also make life a fair bit easier if it works for them as it's about to get a whole lot squishier with #3's arrival.
Vic, that's so cute about DS coming in for a cuddle! That's the other untested aspect of this scenario too... what DS will get up to if he wakes up and wanders at night as he's usually in a cot.

