We started sleeping with DD (2.5 yrs) when she was around 10 months. Similar reasons to what others have said, easier, last resort, etc. Best thing she ever insisted on. She still sleeps with us for part of the night. DS (5 months) has slept with us since birth so 4 in the bed. Bonding between the two siblings is amazing, in the morning the first thing either of them do is look for the other one. When they realise they are both awake they laugh and giggle together. It is the best family time of the day when we all wake up together.

Honestly i don't understand the need/want for it?! maybe im a horrible mum but i believe that the reason my child has a room and a cot is to sleep in it! and i think its part and parcel of them growing up and becomming independant! once they reach a certain age they are no longer babies anymore!
This is my opinion but independence and growing up aren't exclusive to where you sleep. My DD is one of the most fiercely independent 2 years olds you will meet. Toilet trained herself, gets her own brekky, dresses herself etc. At night though, she wakes up and needs the reassurance that her security (her parents) are there. Yes from an adult perspective we are in the same house, but from her point of view, she wakes up, she misses us or is frightened from a dream, she wants a cuddle. Why would I deny her that just because it happens to be dark outside. I would certainly never let her sit by herself for hours on end in her bedroom during the day missing us.

There will come a time developmentally where she no longer needs this reassurance and will sleep in her bedroom all night long. She will also probably tell me I embarrass her and want me to drop her off around the corner from school so no-one sees her daggy mum.