Well Honestly it depends. Islamically there is no such thing as engagement. So if you have to make a commitment islamically Nikah is the way to go. Like others mentioned there are times when certain circumstances causes the couple to wait for certain thing or time till they can start their life and accept responsibility. If they dont do Nikah then there is reallly no commitment there not to mention that if they decide to see or talk to each other in between is haram itself.
So that's why some families chose to do Nikah as a means of a commitment. Nikah is a Commitment or a contract, Marriage is not valid until it has been consumated which is obviously after the bride officially leaves her parents house. Also, to add on Yes there is no such thing in Islam to have a Huge celebration for Rukhsati But there is a Concept of rukhsati, which is the Bride leaving parents house with parent's permission. Or in other words Parents/Guardian handing the bride off to groom. That can be done at home to in a hall that up to the couple. So she really cant run off with the groom between nikah - sending off time period without girl's parents consent.
I had to do research in this topic for my cousion when her in laws wanted a nikah and a year wait and my family was not familiar with this concept. so thats why i know little bit of details.
You are absolutely wrong.. After Nikkah, there is no hurdle, even without rukhsti they can live together.. Nikkah is the only and final thing to make a valid marriage..