Very good points, however in my case I will repeat myself once more:

Me and my Mrs bought the monitors simply because we can hear better what the little 'un is doing upstairs. It's a piece of mind thing that set us back £30, yes only £30. If it reduces the chance of him suffocating or any of the other associated nasties by say 20%, I'll consider that money well spent.
This isn't to say I'm gonna go out and buy EVERY gadget going, no, I just appreciate for such a little outlay what difference piece of mind it makes.

And as for African tribes, I doubt they're living in multiple floor houses made from brick and mortar with doors, internal walls and such to block out noises

 

I have to say that they are a great thing too

We have the Angelcare one with a sensor mat as being a first-time parent we were naturally overly protective. Got it for £45 from ebay and the comfort it gives in our town-house is great.

We can put our baby daughter down in the cot and be sure in the fact that anything untoward and we'll be alerted.

When deciding what to get - we did look at the video ones and were told there's no point - after looking at your baby for a few mins asleep the video is of no use whatsoever!