What bikes do couriers use?

[vanordinaire]

I'm of the opinion that if you want the best of anything in life, buy what the professionals use.
I don't live in a city so have never actually seen a courier in real life but would like to know what bike choices they make and why?
I'd imagine they'd use good quality steel frames for all day comfort,no suspension which would just sap energy, narrow bars for squeezing through traffic,heavier than normal tyres etc or do they just buy as cheaply as possible as the bike is likely to be wrecked/stolen every few weeks?
At 50 plus and 20 stone I'm not up to cycling as much as I used to be so not considering it as a career, just curious.