For myself, I have never used such services, as it easier for me to compose a text myself. However, I know many ppl who do this. Sometimes it makes sense to me - there are classes and tasks that are time-consuming but senseless. Like if u have to write some topics and classes that u don't like and that u'll probably not need in ur professional life. This allows u no invest more time in other things that are more relevant to ur future profession. So... If this service is in demand, ppl need it. But for myself, I wouldn't use it.
There seem to be lots of these companies around so the demand is obviously there. I personally would feel guilty if it wasn't my own work, it would take the shine off my good marks. But then I suppose if you're never going to have to use those essay writing skills in the future then why waste your time trying to perfect them when you can get someone else to do it, its learning to delegate I suppose