What's the point of the ambulance scene in Trainspotting?

He's getting a taxi to the hospital but an ambulance goes by in the next street, is this just to show how stupid it is to get a taxi to the hospital?

Yes, your first answerer has the right idea. Most 'normal' people, when having symptoms like Renton's, would call an ambulance. An ambulance driver would ask questions to determine what Renton had taken - although this cannot be revealed to the police without consent (drugs OD not being the sort of offfence which overrides confidentiality). He could have had some emergency treatment in the ambulance or even where he was, but he and his friends live the sort of life which makes one distrust people in uniforms, so Swanny puts him in a cab instead. Calling a cab just highlights the abnormal and self-destructive nature of the junkies' lives.

No, I think it's to point out that he COULD get an ambulance but Mother Superior wouldn't want his address connected with a drug addict

yes because Paramedics have life saving stuff on there trucks they are a ER on wheels and hence the name MEDIC like a doctor . Para meaning before unless you are like 2 Min's from a hospital and the ambulance is 5 Min's away don't wait. But I have seen people walk through the ER doors that live about 30 Min's away and an ambulance was 10 Min's from them . And that Pt was having a full blown heart attack or as us medical people call MI he died because he did not get the live saving drugs soon enough.They can do procedures like airway management for a person not breathing can start a hurt back . With some drugs Paramedic have almost the same drugs that hospital's have that can save your life . Not the dopey kind they do have pain medication but go by protocol set by a doctor on when they can give and why...