It may be a useful skill, but it is not essential.
There are far too many teenagers who can't sort themselves out, and who are completely helpless about getting anywhere. It's one of my real bugbears.
DH and I both drive, but we cycle, use buses and trains and walk as well. DCs in their late teens have been brought up the same. They are both totally capable of looking up bus and train times, walking two miles to the nearest railway station (we will deliver/collect if it's after dark), cycling a few miles to meet up with friends, etc. They are actually quite scornful of their peers who can't/won't do this. One was pleased to find that she could make quite a long train journey, with four changes, to go to a university open day on her own without mishap. Good for their self confidence.
A friend of theirs who was visiting wanted to go home, had texted parents to ask for a lift and got no response - both parents out doing other things. She actually cried when I said I was sorry but I was about to start work and couldn't take her and she should walk. Bright, sunny day, residential roads, less than three quarters a mile 
I know another girl, aged 17, who has never been on a bus or a train on her own and is convinced she'd be terrified. I blame the parents.
Of course they should contribute, or at least realise that they have to do without other things if they don't have any income of their own. It does not do them any favours to go on babying them about when they get older.
Rant over.