I don't really use IM, but I gave my old iphone to my daughter (5) and bought my other daughter (3) an Ipod touch.
Since they aren't on phone plans, I can't send them SMS, but would like to shoot them an IM every once in a while. Due to their ages, this isn't something I would be doing daily, but maybe once a week, hence the need for one that lets you stay on. I don't want to go on to their phone/touch daily.