Mono chain and seat stays.

[Mos6502]

My old Columbia has a mono chain stay (the chainstays are actually one tube which is formed like an extremely tall U and then joined at the bottom of the U to a short tube which connects to the BB) and I always thought this was peculiar, it seemed to be that this would be given to more side to side flexing than conventional chain stays.
Now I'm seeing a lot of new bikes that have these mono seat and chain stays - are they stronger/lighter or somehow better than the traditional designs? I'm just wondering what the reason behind this is.

[LWaB]

Cheaper to manufacture, mostly.

[Peterpan1]

They are cheaper, some respects they also work out well in situations where the exact part you are after may not be available: really large bikes, possibly some 29er projects, or specialty things like the Bike Friday, or super long stay touring bikes.